I’m not a technical person, so i don’t really know, but from a blog i read about switching from lemmy to piefed, i guess there should be some problems :

I also changed the URL from jemmy.jeena.net to piefed.jeena.net because I don’t want the other instances to be confused when suddenly a different software with different contents is available under the same domain. But I’m thinking to start up Lemmy and unsubscribe from all communities and to delete the one community I have on my Lemmy instance which has only very few subscribers.

That at least would be a somewhat clean way to dispose of this lemmy instance. Oh and I should return a 410 Gone to indicate that it’s permanently gone.

while I don’t know the answer to the question, I am very certain that

or would whole fediverse crash

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.

quokka.au changed from Lemmy to PieFed and it was ok once the remote servers refreshed their public key caches, about a day.

Mastodon to GTS could turn out different but yeah.

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

I changed Ruud.social from akkoma to sharkey. It still works.

I changed kbin.earth from kbin to Mbin, and it's been working great. It definitely helps that Mbin is a fork of kbin.

It definitely depends on the software, for example in matrix you can easily cause federation issues in some cases. this won’t blow up the fediverse, but it could impact the user experience if they rely on or interact with that instance.