@cy @Hyolobrika This is possible because we introduce an additional key controlled by the user. You can read more about this in Identity Proofs proposal: https://codeberg.org/fediverse/fep/src/branch/main/fep/c390/fep-c390.md
When two accounts are linked to the same key, you can migrate your followers from one server to another even without permission from your old server. The migration process is very similar to what Mastodon and other popular AP servers already support, but it is a "pull" rather than "push". See this document for details: https://codeberg.org/fediverse/fep/src/branch/main/fep/7628/fep-7628.md
This was implemented in Mitra but (as I learned in the process) it is too little, too late. As @Hyolobrika pointed out I'm now working on FEP-ef61 that will make all content portable. It won't be necessary to "migrate" anything anymore because the location won't matter