@thejacenallen yeah....? It's exactly like email. You're not changing your email once you make it unless you're doing some complicated thing. It's been like that for...ever.
You can accomplish this using several softwares from the Facebook side of the fediverse (Friendica, Hubzilla, Streams, Forte, and possibly a couple of Threadiverse projects). It appears you're using Mastodon, which means you can't.