in case anyone is curious about the underlying cause of it being impossible to migrate a mastodon server from one domain to another, this limitation is due to the activitypub protocol itself, which (simplifying for brevity) uses the equivalent of absolute/hardcoded URLs for posts
this fediverse enhancement proposal allows for relative domain names. given that it was written a year ago, and with the molasses-like development the protocol has, i sadly suspect it's unlikely to be integrated any time soon.
more here:
https://codeberg.org/fediverse/fep/src/branch/main/fep/e3e9/fep-e3e9.md
