Re-federation is probably possible. BUT! You're going to always have problems with older content. Case in point my federation error messages is at 2300. About half are failed requests on fmhy.ml.
So for re-federation what's needed:
1: Remote instances should unsubscribe all users from any fmhy groups. They're dead now. They can only announce that and hope they do. I reckon when their errors start ramping up (as I saw yesterday) they will be looking into why. Probably to help de-federate from the old URL
2: The fmhy instance should unsubscribe all users from all remote groups but keep a note of the groups while identifying as fmhy.ml. Then once on a configuration for the new domain re-subscribe to each one. The first step should hopefully stop them trying (and failing) to federate new events to the old URL. The second step should trigger federation with the new one.
3: They could be able to keep the DB. But I am not sure in what places the old domain might be stored in the DB and what would need fixing there. Also not sure if they'd need to regenerate keys. Not sure if they'll see the key was attached to the old domain and refuse to talk to the instance.
Now what's going to be a problem? Well ALL the existing content out there has references to users on the old domain. It's VERY hard to fix that. Like every instance would need to fix their database. Not worth it. But, whenever someone likes/unlikes or comments or whatever a post made from fmhy.ml then there's a good chance a remote instance will queue up a retrieval of:
1: User info about the poster/commentor/liker
2: Missing comments/posts for a like/comment event
And those will fail and error log. I don't think there's a way around that aside from editing the whole database on every instance. Again, IMO not worth it.
Would be a nice federation feature if, provided you could identify with the correct private key, announce a domain change which would automatically trigger the above in federated instances, or at the very least some kind of internal redirect for outgoing messages.