For example I would selfhost Mastodon with me@fedi.com but then decide to change the server software to GoToSocial but keep the same me@fedi.com. Would everyone who was following me@fedi.com keep me in their follower list or would whole fediverse crash?
while I don’t know the answer to the question, I am very certain that
is not it; if that could make the whole fediverse crash, someone could do it maliciously. At most it will cause problems delivering your messages or delivering messages to you.