When an Adult Gold Dragon changes into an Archmage, what are its saving throw and skill bonuses?

Question

An Adult Gold Dragon has Change Shape as one of its actions (emphasis mine):

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own [...]

In a new form, the dragon retains its [...] proficiencies, [...] and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

With this action, the dragon can magically polymorph itself into an Archmage. While the dragon is an Archmage, what are its saving throw and skill bonuses? (To be clear, I'm not talking about its spell save DC here.¹) Does the fact that its proficiency bonus is different in Archmage form have an effect on these bonuses? Similarly, do changes in the dragon's Strength, Dexterity, and Constitution scores caused by being an Archmage matter?

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¹ It probably wouldn't have a spell save DC, because non-innate Spellcasting is a class feature.

Answer 1

What are proficiencies?

The dragon retains its proficiencies, but what counts as a proficiency? In the Character Creation chapter, a character's proficiencies are defined as follows:

You also gain a number of proficiencies: armor, weapons, skills, saving throws, and sometimes tools.

The term is bolded in the original text, as this is the place where it is first defined. A couple sections later, the concept of a proficiency bonus is introduced (emphasis mine):

Your proficiency bonus applies to [...]:

• Attack rolls using weapons you're proficient with
• Attack rolls with spells you cast
• [...]

Your class determines your weapon proficiencies, your saving throw proficiencies, and some of your skill and tool proficiencies. [...] Be sure to note all of these proficiencies, as well as your proficiency bonus, on your character sheet.

Proficiency bonus is not mentioned in the list of proficiencies above, and it is noted separately from the proficiencies themselves. Therefore, a character's proficiencies indicate instances where the proficiency bonus is added, but the bonus itself does not count as one of the character's proficiencies. When a character reaches level 5, their proficiencies (usually) do not change; instead, their proficiency bonus increases to +3.

Does this apply to monsters?

While monsters have many abilities unavailable to PCs, they are typically built using the same set of rules. The introduction to the monster rules refers back to the player rules in the sections for Size, Alignment, Hit Points, Speed, Ability Scores, etc. What about saving throws and skills? Are monsters proficient in them?

The Skills section comes right out and says it (emphasis mine):

The Skills entry is reserved for monsters that are proficient in one or more skills. [...]

A skill bonus is the sum of a monster's relevant ability modifier and its proficiency bonus [...]

The Saving Throws section almost says the same thing:

The Saving Throws entry is reserved for creatures that are adept at resisting certain kinds of effects. [...]

A saving throw bonus is the sum of a monster's relevant ability modifier and its proficiency bonus [...]

To me, the intent is clear: monster saving throw bonuses also count as proficiencies, much like a PC's class saving throw bonuses do.

Comparison

When using Change Shape, an Adult Gold Dragon retains its proficiencies. I will compare its saving throw and skill bonuses in dragon form to those in Archmage form.

Adult Gold Dragon

The dragon's proficiency bonus is +6, because it is a CR 17 creature. Creatures of CR 1-20 have the same proficiency bonus as PCs of the same level.

• DEX +8. The dragon's Dexterity (+2) plus the dragon's proficiency bonus (+6).
• CON +13. The dragon's Constitution (+7) plus the dragon's proficiency bonus (+6).
• WIS +8. The dragon's Wisdom (+2) plus the dragon's proficiency bonus (+6).
• CHA +13. The dragon's Charisma (+7) plus the dragon's proficiency bonus (+6).
• Insight +8. The dragon's Wisdom (+2) plus the dragon's proficiency bonus (+6).
• Perception +14. The dragon's Wisdom (+2) plus double the dragon's proficiency bonus (+12).
• Persuasion +13. The dragon's Charisma (+7) plus the dragon's proficiency bonus (+6).
• Stealth +8. The dragon's Dexterity (+2) plus the dragon's proficiency bonus (+6).

Of particular note is the dragon's Perception bonus. Much like a rogue with Expertise, the dragon adds double its proficiency bonus to Perception checks. The Skills section tells us about this possibility:

For instance, a monster might have a larger-than-expected bonus (usually double its proficiency bonus) to account for its heightened expertise.

Adult Gold Dragon → Archmage

The Archmage's proficiency bonus is +4, because it is a CR 12 creature. When an Adult Gold Dragon uses Change Shape to become an Archmage, its saving throw and skill bonuses are as follows:

• DEX +6. The Archmage's Dexterity (+2) plus the Archmage's proficiency bonus (+4).
• CON +5. The Archmage's Constitution (+1) plus the Archmage's proficiency bonus (+4).
• WIS +6. The dragon's Wisdom (+2) plus the Archmage's proficiency bonus (+4).
• CHA +13. The dragon's Charisma (+7) plus the Archmage's proficiency bonus (+4).
• Insight +6. The dragon's Wisdom (+2) plus the Archmage's proficiency bonus (+4).
• Perception +10. The dragon's Wisdom (+2) plus double the Archmage's proficiency bonus (+8).
• Persuasion +11. The dragon's Charisma (+7) plus the Archmage's proficiency bonus (+4).
• Stealth +6. The Archmage's Dexterity (+2) plus the Archmage's proficiency bonus (+4).

In general, during Change Shape a dragon's saving throws and skills are weakened; however, there are exceptions. A Githzerai Anarch (MTF, p. 207) is a CR 16 creature (+5 proficiency bonus) with 21 Dexterity (+5), raising the dragon's Dexterity save bonus to +10.

Rules-as-written, the decision of whether a proficiency bonus should be omitted, included, or doubled is maintained, while the proficiency bonus itself changes to match the creature changed into.

Answer 2

Claim: A dragon cannot shape change into an Archmage (or any other NPC class).

Argument: The source books that define monsters/races have (unfortunately) used the same stats block for defining three separate kinds of entities:

1. Races (e.g. Bugbear, Beholder, Gold Dragon)
2. NPC classes (Commoner, Tortle Druid, Archmage)
3. NPCs (Demogorgon, Graz'zt)

Only races are applicable to ShapeChange (not NPC classes or NPCs).

As the OP mentions (new emphasis mine):

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own [...]

In a new form, the dragon retains its [...] proficiencies, [...] and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

NPC classes like Tortle Druid (MToF-242) are a convenient means of bundling a race and class together, while NPC classes like Archmage don't even bundle the racial aspect (the race is "any humanoid") and are simply a means of summarizing "representative class features". Such stats blocks are NOT suitable targets for a dragon's Change Shape ability because class features are not acquired.

Supporting Evidence: The 9th level Shapechange spell says:

You assume the form of a different creature for the duration. The new form can be of any creature with a challenge rating equal to your level or lower. The creature can't be a construct or an undead, and you must have seen the sort of creature at least once. You transform into an average example of that creature, one without any class levels or the Spellcasting trait.

If a 9th level spell doesn't allow you to acquire class levels, I contend that dragons also do not get class levels from their change shape ability. More generally, any race with the ability to change shape (Imps, Doppelgangers, Succubi, Death Slaads, etc.) can change into other races (per the limitations in their description) but do not get class levels as part of that change.