He has another post here:

https://old.reddit.com/r/Lemmy/comments/1jgvlof/having_problems_changing_my_matrix_id_username/

I don’t actually understand the issue that user is having.

It sounds like he had one Matrix account. That got compromised. He set up a new Matrix account. He intended to change his Matrix account to indicate that his username is kevinterribly@lemmy.today rather than kevinterribl@lemmy.today.

Both of those two above accounts exist on lemmy.today, were recently created, and I assume by him.

It sounds like Matrix supports some kind of account portability from one Matrix instance to another.

https://matrix.org/docs/communities/switching-providers/

It sounds like one needs to indicate a Matrix username and a Matrix instance when doing this.

I’m guessing that he’s trying to use his lemmy.today account there.

I think maybe he doesn’t realize that Matrix has nothing to do with the Fediverse, thinks that it’s a Fediverse protocol and that one chooses a Fediverse account and then uses it as some kind of global identifier everywhere, and that’s the source of confusion. Like, he thinks it’s comparable to a Google single-sign-on account, where he’s choosing a username on lemmy.today and then wants to use that to sign into Matrix.