Changing the preferredUsername of an actor seems to work ok with Mastodon in my testing:
Maybe I’m missing something though! Looking at my video again, I notice that the record id of the Mastodon account is actually changing, suggesting the old one is being replaced, instead of being updated, as Clare alluded here perhaps
Nevertheless, it still seems to work? Content associations, follow relationships etc all seem to remain in tact. Am I missing some issue with this?
Note that this Discourse feature (i.e. allowing you to change the preferredUsername) is currently in a draft PR: