depends on the fedi software involved

"Yes. As long as you don’t delete your old account, and as long as the server it is on is still functioning, you can always go back to your old account and cancel the redirect."

https://fedi.tips/transferring-your-mastodon-account-to-another-server/

I just saw the first instance I was on and it occurred to me. What if I signed up again?

I guess the account still exists so I'd have to use the log in details I used.

Yeah, if you log back into the original server you'll be taken to the settings page and you'll see a banner that says "Your account is inactive because it is currently redirecting" and then it gives you the option to cancel the redirect