It's not, unfortunately. The main issue I see with implementing that is all of your existing posts/comments would still link to your old account on other instances. So either your old instance would have to forward requests for your account to the new instance, or maybe ActivityPub has some way to push that update out and update your account's home across the fediverse. I don't know enough about it unfortunately.
You are welcome to create multiple accounts in any case. Suppose that defeats the purpose of the fediverse though.
Very true, if you don't mind losing your post history. A simple way to migrate subscriptions would be great for those folks that make a new account every once in awhile anyway.