AFAIK they would remain tied to your old account. The instances copy everything off each other. So if reddthat.com would go down, your comment would still show as EndlessNightmare@reddthath.com If you were to make a new account on a different instance it would not have your posts and comments “attached” to it.

This could be an issue if you rely on reputation for something that is tied to your account. In that sense having some mechanism to proof the old account to be owned by you would be good.