Mojang to Microsoft migation Minecraft username issue

The username that you said is for Java edition?
Or do you mean the gamertag from the Microsoft account?

The issue is that that the username is mine. I've had this username for years. I haven't changed it ever. Up till just before I migrated, I had that username. When I migrated, I now apparently don't. No one else should be able to have got it.

Probably the system doesn't notice a difference between capital letter M and m

Sincerely in this kind of situations there is not much that we can do, you can try using a similar username, or add it a single number at the end

Yes, for Java edition. My gamertag for microsoft doesn't have a capital M. Also to note, the account i'm trying to use is also java edition, not something different.

Hi and thanks for reaching Microsoft Xbox gamer's community. My name is Josef.
I'm an Independent Advisor and a Microsoft Xbox user like you.

Hope you're doing great today Thomas

Sadly if the system is telling you that you're not able to use that specific username is not possible to do anything, is already claimed