So, since there are no "official" rules for retraining. Our GM might like some possible suggestions.
How would you handle it in your campaign if a player wanted his PC to retrain a couple of proficiencies and/or a feat?
Find a way to get the friends cantrip maybe?
That is the response a lot of people have, but I find it to be very useful for certain PCs. I get what I want, and then I take the repercussions.That would be suicidal because Friends has the unfortunate side-effect of having a duration of 1 minute, and immediately after that, the target automatically, 100%, knows you naughty word with them with magic, and the spells specifically calls them out as being angry about it. It's one of the worst cantrips in the game as a result. 1 minute isn't even enough time to get clear of most social situations even by anything short of sprinting!
Friends might serve that PC to delay a combat for a minute rather than have it start as soon as they are encountered.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.