Cant use my old username from an account that cant play minecraft anymore (Java edition)

i dont know if this affects every launcher, but i used to have gamepass on my old account, which gives me access to bedrock and java edition (as long as i have the subscription). I recently changed which of my 2 accounts have gamepass, so my old account doesnt have bedrock or java anymore, when i went to change my username on my old account, it wouldnt even show me my old username, but if i tried to use the same username on my new account, it would say the username is taken. And since im unable to change my old username, its forcing me to change my new one. Someone please fix this so that i can somehow access my old username on my new account without deleting the entire microsoft account.