Well currently an admin could easily intervene and stop a migration by removing the community mods, to be fair.
For example, now that I am working on an AP server, I can take all your posts on !television@piefed.social and mirror them on !television@metacritics.zone. I could also avoid sending notifications to you, so you'd be aware of this only if you visited the site directly. How would you feel about that?
I mean you could just copy my posts anyway manually, if you were so inclined. There wouldn't be much I could do about it no matter how you did it.