1. I create an account @ploum@writing.exchange on writing.exchange.
2. I go to mamot.fr and, in the settings, I enable migration to @ploum@writing.exchange.
3. I go to writing.exchange and, in the settings, I start the migration from @ploum@mamot.fr.
All my followers and following are automatically transfered. For them, it is transparent. They still follow me on my new account without them being even notified.
Of course, you need cooperation from mamot.fr. If mamot.fr decide to close your account, you can’t migrate it.
But it works well, I’ve used it myself. It is really great and allows people to do "server hoping" to join a community that fit better their need.