You can't import your posts, but they will still exist at your old account: if I move from foo@social.example to bar@social.example, then all the posts I made as @foo are still visible on @foo's timeline, rather than getting moved to @bar. (You would have two copies of all your posts otherwise, since people can still fav/boost/interact with the post made when you were still @foo.) Moving accounts will redirect your followers to the new account, and you can import all your old follows/likes/&c to the new account.