This is a critical feature missing. Especially considering when one instance goes down, so does your account.
If announced, we should be able to easily transfer the account.
But an instance might also go down unannounced (especially now with so many hosting instances just for fun or to experiment with it).
While in such a case your posts and comments on this local instance will be gone, there needs to be a way to recover your account on another instance with all your posts and comments made on other instances still connected to your account, being editable or deletable.