For example I would selfhost Mastodon with [email protected] but then decide to change the server software to GoToSocial but keep the same [email protected]. Would everyone who was following [email protected] keep me in their follower list or would whole fediverse crash?
it’s messy but doable. if you make sure to keep all actor keys unchanged and insert all your followers manually, it should not matter. remotes may still show you as a mastodon instance for a while, but eventually will refetch the nodeinfo.
if you rotate keys, you will still receive activities from those following you but your posts will be rejected as the http signature doesn’t match the pubkey remotes have of you. eventually software should refetch actors and pull your new keys
if you don’t replicate your followings locally, you will have to re-send follows manually and these may get discarded (httpsignature issue) or discarded (software thinks the relation already exists and thus do nothing)
with enough time it should fix itself: around 2 years ago i switched from mastodon to akkoma on the same domain and didn’t do these things. took a few months for stuff to fix itself, but it eventually did