This has come up over the years: Update quotes and mentions when username is changed - #10 by sam
Back in 2016 @eviltrout was against it, not sure where he is sitting on it these days.
Anyway, I have a workaround in Discourse in this PR:
https://github.com/discourse/discourse/pull/11540
It will handle the facebook new signup issue by downcasing any username with sigma upfront. This means all you need to do is fix Spiros username and any other users with final sigma by downcasing and the issue should be gone long term.
(PR merge pending)