The old server broadcasts a 'this person has moved'-message. Your followers will then automatically switch over to the new account.

This implies that:

- If the old server is offline or unwilling, you cannot move - Your old toots and interactions will still be tied to the old address, so you leave those behind.

There is some talk going on to find ways to mitigate this and it might be possible to fix the second issue.

The first issue is a bit trickier though. If you were to remove trust from the server, it becomes way harder to fight spam.