After 10+ years on Reddit with the same username, I signed up for Lemmy without the underscore back in June. Then I stopped using all social media until Sync came out and now my password/email recovery don't work. So I effectively locked myself out of my real username and back into the same shittier one that I had to use on Reddit :'(
I'm pretty sure usernames, like community names, are on a per-instance basis. So mantissa@lemmy.ml or mantissa@sh.itju.st.works or whatever it's called might be available