I’m not a fediverse developer but as I understand it the activitypub spec identities people and communities by their domain, and does not define a way to mutate.

Translation: you would loose all your peers and your comments.

If lemmy.ml copied the databases to a new domain, then when websites start to peer again there would be duplicate comments, one from the old domain and one from the new domain.

Translation: it would require the cooperation of every instance to be clean

I’m not a fediverse developer again, this could be wrong