This is an experimental feature
Friendica will recreate your account on the new server, with your contacts and groups. A message is sent to Friendica contacts, to inform them about your move: If your contacts are runnning on an updated server, your details on their side will be automatically updated.
Contacts on GNU Social will be archived, as we can’t inform them about your move. You should ask them to remove your contact from their lists and re-add you, and you should do the same with their contact.
Newer Diaspora servers are able to process “account migration” messages.