Is skill retraining a legal option in 5th edition D&D?

Question

When a similar question was asked on this site, I interpreted the original phrasing of the question as a question regarding the retraining of skills, rather than the retraining of classes / class features.

I ran some quick searches in the database of questions that have been asked on this site, and I found that there's isn't a question that addresses precisely the possible options available to a DM regarding skill retraining as specified in current D&D 5th edition literature.

While reading that first question, I came up with an answer to the question in the manner that I interpreted it. However, I don't believe that my answer (provided below) is entirely exhaustive in describing the ways that a DM can allow for skill retraining 5th edition D&D. What are some additional ways?

Answer 1

It is now possible using Tasha's Cauldron of Everything

Although the answers above were correct at the time they were posted, they no longer are. Since Tasha's Cauldron of Everything was released in 2020, it has been allowed for a character to retrain a skill (although DM approval is necessary). On page 8, we find the section labeled "Changing a Skill":

Sometimes you pick a skill proficiency that ends up not being very useful in the campaign or that no longer fits your character’s story. In those cases, talk to your DM about replacing that skill proficiency with another skill proficiency offered by your class at 1st level. A convenient time for such a change is when you reach a level that grants you the Ability Score Increase feature, representing that your character has spent a level or two studying the new skill and letting the old one atrophy.

Answer 2

Is skill retraining explained in any of the 5th edition D&D core rulebooks?

From the reading that I've done, there is no explicit explanation of a "retraining" mechanism in 5th edition D&D.

Other Courses of Action

1. The Skilled Feat

A way to avoid undoing skill choices is to make sure that you can train more skills after Level 1. Conveniently, there is a feat called Skilled that gives players the options to train themselves in three more skills than they are trained in through their class and race. Considering how often skills are utilized for the group's benefit in D&D, this is a very viable feat to have a character take. The feat is described as follows (PHB, pg. 170):

You gain proficiency in any combination of three skills or tools of your choice.

2. Learning from a Willing Instructor

The Player Handbook describes the manner in which a player's character may learn a new language or tool, a concept which could reasonably be extended, if the DM sees it appropriate, to include new skills as well (PHB pg. 187):

First, you must find an instructor willing to teach you. The DM determines how long it takes, and whether one or more ability checks are required. The training lasts for 250 days and costs 1 gp per day. After you spend the requisite amount of time and money, you learn the new language or gain proficiency with the new tool.

As I said, the PHB never specifically mentions that learning new skills can be done as a down-time activity. You could reasonably apply the above rule to allow players the option of learning skills from one another. Judging from the wording of the Skilled feat, described earlier, the PHB indicates that proficiency with a certain set of tools, which are specifically allowed to be taught and trained through a willing instructor according to this excerpt, and proficiency in a certain skill are valued at roughly the same level.

3. Multi-classing for Additional Skills

Under the "Proficiencies" sub-header in the "Backgrounds" section, the PHB sets forth a useful rule to keep in mind on "double-dipping" proficiencies / skills (PHB pg. 125):

If a character would gain the same proficiency from two different sources, he or she can choose a different proficiency of the same kind (skill or tool) instead.

"A different proficiency of the same kind" seems to indicate that the character is allowed to train in any of the list of proficiencies provided on the player's character sheet as a substitute for their "double-dipped" proficiency. For all of the below multi-classing options, it is important to remember that if you multi-class into a class that offers many of the same skills as your original class, you are allowed to substitute those "double-dipped" skills you are forced to take with any other skill on your character sheet.

Short story shorter, if you want to train in as many skills as possible, then it is best to make sure that your character's original skill choices overlap heavily with the skill choices offered by the class you are multi-classing into (particularly, Rogue or Ranger) so that you have the whole list of skills available to train your character in, instead of just that class's predetermined set of skills.

Bard

Training your character as a 1st-level Bard allows your character to train into three new skills. Further, selecting the College of Lore archetype allows your character to train into three additional skills.

Rogue

Training your character as a 1st-level Rogue allows your character to train into four new skills, selected from a predetermined set of skills.

Ranger

Training your character as a 1st-level Ranger allows your character to train into three new skills, selected from a predetermined set of skills.

Answer 3

As I mentioned in my answer to the other question:

The PHB (p. 187) presents training in a tool or language as a possible downtime activity. It says, "Your DM might allow additional training options." It takes about as long as a public school year in the US to complete one training option.

[...]

The DMG (p. 231) also presents rules for granting training as an "alternative reward." That means this option is presented as an alternative to magic items, so you're not going to be able to just go out and do it. You can spend the same amount of downtime that the PHB requires for training to learn a new skill, gain a feat, or gain inspiration daily for about a week. You'd have to ask someone else how 7-10 days of inspiration compares to a permanent skill or feat, let alone would be worth the 9 months of investment to get it.

There are, however, no means in the core rule books to forget a currently learned skill (unless you count polymorph).

As the other answer in the other question states, however, in Adventurer's League you're allowed to completely rebuild your character any time before level 5. The only thing you can't change is your race or faction. That's not a D&D rule, however, but a campaign option for Adventurer's League.