Minecraft Java Edition Username stuck on second account using Xbox gamepass that got...

Hi, I'm here asking for help, a few months ago I bought an Xbox GamePass using my second Microsoft account to play Minecraft, months later, I finally bought the full Minecraft on my main account, I thought my username had been deleted when the Xbox GamePass expired. I was asked to enter a new username into my main account. I tried my old username, but it didn't work, because someone had already claimed it, I was surprised that all my server data used my username to save my progress. Can someone help me how to replace it? I don't want to waste my money for nothing. I've tried changing it, but can't, looks like I'll have to buy it to change the name.

:)