Can't disable my steam (Don't use anymore) from cross-save.
I changed my name once but Bungie didn't get that, so Bungie shows the first Steam name rather than the new one. I can still access my account and I just get an error code when trying to authenticate the account. I have to use the new name to log in, the old name that Bungie thinks I have I can't log into even though they are the same account.
Changing your Steam Display Name would have absolutely no impact on your BungieName, nor does it change the Steam ID. If you want to change your BungieName, you can do so from your Bungie.net settings.
However, if your iss is that you created a completely new Steam ID, then it is, indeed, different from your other Steam ID. In order to disable Cross-Save and unlink an account, you must be able to authenticate to it. If this is the case, I'd suggest using Steam's account recovery system from the website login page to recover the account username.