For example, if a 17th level character with a +6 proficiency bonus and who isn't proficient in Acrobatics casts the shapechange spell to turn into a Quickling (who have a +6 Dexterity modifier and an inherent +8 skill bonus in Acrobatics), would the character's new Dexterity (Acrobatics) bonus be +8 or +12?
In your Quickling Example it’s +8
Taking your Quickling example, you are not proficient in Acrobatics, but the Quickling is, so you take the Quickling’s skill value for Acrobatics of +8.
Rules for Shapechange
Shapechange can be cast by Druids and Wizards (because it is on their spell lists), and by Bards (if they choose it as one of their Magical Secrets at 18th level)
The relevant portions of the Shapechange spell are:
Your game statistics are replaced by the statistics of the chosen creature, though you retain your alignment and Intelligence, Wisdom, and Charisma scores. You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature. If the creature has the same proficiency as you and the bonus listed in its statistics is higher than yours, use the creature's bonus in place of yours.
You retain the benefit of any features from your class, race, or other source and can use them, provided that your new form is physically capable of doing so. You can't use any special senses you have (for example, darkvision) unless your new form also has that sense.
There are some important implications of this wording:
- If both you and the creature are not proficient in the skill or saving throw then you use whichever is relevant
- if its related to INT, CHA or WIS you retain your calculated stat in the skill or saving throw
- if it’s related to STR, DEX or CON you gain the creature's stat in that skill or saving throw (ie the creatures relevant score modifier if it’s not explicitly listed)
- If you are proficient in the skill or saving throw and the creature is too you take whichever skill/saving throw total is better (yours or the creatures)
- importantly you do not get to use your proficiency bonus in place of the creature’s when doing this comparison.
- If you are proficient in the skill or saving throw and the creature is not you retain your own value for that skill or saving throw
- If you are not proficient in the skill or saving throw and the creature is you gain their proficiency (ie you take their listed number from their stat block)
- you take the number directly from the creature’s stat block without modification
For most applications of the spell this is the end of the requirements.
As a note Jeremy Crawford has given a ruling on Twitter on how the inheritance/adoption of proficiency works for the Shapechange spell:
While you're under the effect of the shapechange spell, you use your proficiencies, including your proficiency bonus, except when the stat block of the new form has a modifier (proficiency bonus + other modifiers) that's higher for a proficiency you have. #DnD
This supports the comparison operations we have laid out above.
Interaction with Expertise
Creatures don’t have expertise, characters do. Expertise is a class feature of a Bard or a Rogue which allows them to double their proficiency bonus in certain skills.
As a result you will retain your expertise in that skill.
If you have Wizard or Druid they will have had to multi class into Rogue or Bard for one or more levels.
If you have a Bard, they could come into this confluence of rules provided they chose Shapechange as one of their Magical Secrets at 18th level.
The only situation where this could cause an issue is where both you and the creature are proficient in the skill (you can’t have expertise in a saving throw)
In order to apply the spell effects properly, you need to compare your skill total (before expertise is applied) to the creatures skill total.
If yours is higher you take your skill total (including expertise).
If the creatures is higher then you need to figure out what the creature’s proficiency modifier is and recalculate the skill value using the creature’s stat modifier and doubling the creature’s proficiency bonus.
Can you cast Shapechange on other characters?
The range of shapechange is Self.
What does “bonus” mean in the spell?
The game uses the word bonus to describe both proficiency bonus and the total value of skill/saving throw to be added to the d20 roll.
His attack bonus is his Strength modifier (+3) plus his proficiency bonus (+2), for a total of +5.
(PHB pg 15)
Pay attention to your skill proficiencies when thinking of how you want to interact with an NPC, and stack the deck in your favor by using an approach that relies on your best bonuses and skills.
(PHB page 186)
Is Shapechange similar to any other game features, and are there applicable rulings for those features?
The game feature Shapechange is most similar to is the Druids Wild Shape feature (they even include the same text block on character stats).
As a result any rules implications for Shapechange or vice versa would likely also arise in relation to Wild Shape.
The issue arises from the wording in the Shapechange spell, specifically this section:
Your game statistics are replaced by the statistics of the chosen creature, though you retain your alignment and Intelligence, Wisdom, and Charisma scores. You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature. If the creature has the same proficiency as you and the bonus listed in its statistics is higher than yours, use the creature's bonus in place of yours. You can't use any legendary actions or lair actions of the new form.
There is another game feature which has a similar text block, Wild Shape:
Your game statistics are replaced by the statistics of the beast, but you retain your alignment, personality, and Intelligence, Wisdom, and Charisma scores. You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature. If the creature has the same proficiency as you and the bonus in its stat block is higher than yours, use the creature's bonus instead of yours. If the creature has any legendary or lair actions, you can't use them.
They are identical except for the change of “beast” to “chosen creature”
A druid in beast form uses his or her proficiencies, except when the beast has the same proficiency with a higher bonus.
The intent is that the druid uses the bonus in the beast's stat block for any proficiency the druid lacks.
Is your proficiency bonus a class feature?
The table that appears in your class description shows your proficiency bonus, which is +2 for a 1st-level character.
(PHB pg 12)
It is explicitly part of your class by being included in every class description.
How is this relevant?
As a result your proficiency bonus carries over between forms for skills you are proficient in.
For skills you are not proficient in you gain the creature’s proficiency (and their proficiency bonus).
For skills you are both proficient in you take whichever is better.
Yes, you add your own proficiency bonus. For your example, it would be +12.
Check for yourself on D&D Beyond. Here is a similar case, a level 20 Druid wildshapes into an Ape, who originally had +5 Athletics. The Druid now has +9. This was built on D&D Beyond, you can try it yourself on a char sheet, on the right-side, clicking "Extras".
This answer is based on the Rules as Written [RAW] and on D&D Beyond (as confirmed by Bash). It is in agreement with this. Twitter, as of 2019, is no longer official, and Sage Advice doesn't clarify this topic, but Crawford also follows this logic in Dragon Talk.
Starting with basic rules,
To make an ability check, roll a d20 and add the relevant ability modifier. Proficiency in a skill means an individual can add his or her proficiency bonus to ability checks that involve that skill
When you get new stats (and new modifiers), you have to recalculate the bonuses of your skills. You get to keep your Proficiency Bonus, and add it to the skills you or your new form are proficient with.
From Crawford's Dragon Talk podcast (0:12:00):
If you and the creature have the same proficiency, you use whichever bonus is higher. Let's say both are proficient with the Stealth skill, but the creature's bonus is higher. This beast is actually better at sneaking around than you are, then use the beast's stealth bonus, not yours. So you get to benefit from that increase. It works the other way around, if you are sneakier than the beast. You get to use your proficiency bonus, but you do use the creature's dexterity modifier. Use your proficiency bonus for anything where you're both proficient, but only if yours is higher, but you use the physical stats of the beast.
Let's imagine you have these statistics and proficiencies.
Let's also imagine youre Shapeshifting into a creature with these statistics (this is similar to a Quickling, but with more INT to showcase a point):
Your game statistics are replaced by the statistics of the chosen creature, though you retain your alignment and Intelligence, Wisdom, and Charisma scores. You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature.
Pay attention to the wording: you retain proficiencies, not bonuses.
So, your statistics are now the creatures, but keeping your INT, WIS and CHA scores, and you also get to keep your proficiencies in Saves and Skills! Add your +6 PB when adequate, otherwise use just your ability modifier.
Now keep in mind the following part of the spell:
If the creature has the same proficiency as you, and the bonus listed in its Statistics is higher than yours, use the creature's bonus in place of yours.
Note that this refers to bonuses, not proficiencies. It also only applies when the creature has better bonuses than you, not the other way around.
So, the new form's History bonus is +6, but the Changeling previously had a +8. You're both proficient with it, so you now take the original +8 bonus.
You only use the Changeling's original skill bonus if
- the Changeling had a better bonus than you now have in some skill, and
- you're both proficient with it.
This method follows the RaW, step by step, and has the advantage that nearly all values in your character sheet make sense when you look at them.
As asked in comments, if you had Expertise in Stealth (a class feature that you keep), then in the new form you keep Expertise there. With +6 DEX and +6 PB, you have +18 Stealth (+6 PB +6 PB +6 DEX).
RAW, I'd say yes - and use +12 as a bonus in your example.
From the spell description :
You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature.
You gain the creature's skill proficiencies, not proficiency bonus.
Conveniently, it avoids the need to find/calculate the creature's proficiency bonus, and to argue wether the "hidden expertise" some creature have should be considered as a bigger proficiency bonus.
Back to the spell :
If the creature has the same proficiency as you and the bonus listed in its statistics is higher than yours, use the creature's bonus in place of yours.
Let's use a logical equivalent :
Don't use the creature's bonus if you don't have the same proficiency or it's bonus is lower.
(If you feel this is not equivalent, consider using a truth table first.)
Put simply : Don't look at the block stat bonus if you're not proficient in the first place.
I suspect the initial intent to be something like "You must be proficient in a skill to benefit from a creature's expertise in it." It would make sense. Moreover, had the intent been to use the stat block "as is" most of the time... the wording would have been much simpler.
RAI is harder to get clearly - until you look at DnD Beyond
I have long been focused on designer's feedbacks (compilation can be found here) - which didn't help, as they often contradict themselves.
The DnD Beyond implementation, on the other hand, had to go through a detailed specification & reviewing process before it came live ; after nearly 1 year, we can assume the proper rules have been implemented - and they use exactly this logic.
You can refer to Bluemoon's answer (had been deleted at time of posting...), or see this test character, clicking "extras">"Giant Octopus" on the right to check for yourself the mechanics. Stealth & Perception scores confirm you use the Character's proficiency bonus, and disregard the creature's stat block value if you have no proficiency.