I was less thinking “they might become refederated and have content reach instances that wanted to defederate” and more “what happens at a protocol and database level if one just changes a domain name, updates the X.509 certificate, and then refederates?” I mean, I don’t know how other instances deal with that, whether it can create problems. IIRC from past discussion, there are user keys somewhere in the protocol, and I assume that those would now be identical and collide across old and new instances, for one…