In theory, nothing prevents you from changing usernames.
However, doing this is not compatible with Mastodon. Indeed, Mastodon uses the username + domain pair (the acct: URI, really) as the authoritative identifier, not the ActivityPub id. This pre-dates ActivityPub and it’s one of my main gripes with Mastodon, but unfortunately it’s unlikely to ever get better, as it is too deeply ingrained in both its UX and database design…
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