You lose all your followers. It's like going from blacklight@aol.com to blacklight@gmail.com. It's on you to go to you 1M followers and beg them to refollow you. The goal is to go from tmobile to verizon and your grandma can just call you.
On Mastodon people who see your old profile would get a screen like "this profile has moved here".
Ok, it's still on them to follow you on your new profile, but it's not like you have no connection between the old and new profile.
And I'm among those who want to improve this feature, so actual data exchange (followers and posts) can happen before the two instances during the migration.
