Oh my god we finally got it. On the page I linked earlier, select the "chat with us" button, now hopefully you get on a page telling you to wait and/or manage your other cases. DON'T LEAVE THAT PAGE. Just wait there and after a while the chat will finally pop open and the operator is really quick to solve your issue. Here's is the chat transcript for reference:
Chat Started: Sunday, October 22, 2023, 23:03:03 (+0530)
Agent Sunita K
( 2s ) Sunita: Thank you for contacting EA HELP, my name is Sunita, may I start with your first name please?
( 7s ) Francesco: Francesco
( 45s ) Sunita: Hello Francesco, I am truly delighted to meet you. Hope you are having an amazing day. How may I serve you today?
( 1m 10s ) Francesco: Hi and thanks for your help. I'm trying to link my new Xbox account to my EA account.
( 1m 42s ) Francesco: But 10 years ago apparently I've linked a different account, so now I can't use my new account on this EA profile.
( 2m 5s ) Francesco: Is there a way to just unlock this? I've already unlink my old account.
( 2m 15s ) Sunita: Alright, it seems you need help in linking Xbox, am I correct?
( 2m 19s ) Francesco: Yes
( 3m 21s ) Sunita: Alright, I will help you in linking Xbox. May I know the email linked to the account?
( 3m 49s ) Francesco: To the xbox account? *******@*****.***
( 4m 32s ) Sunita: Thanks, may I know reason of linking new Xbox please?
( 5m 3s ) Francesco: Because is my new account and is linked to my Windows account. I've bought content on it.
( 6m 7s ) Sunita: Okay may I know the reason of unlinking old one?
( 6m 30s ) Francesco: I don't access to it anymore
( 6m 41s ) Sunita: Alright. I can help you in linking another Xbox to this account as one time, it should not be linked to any other EA Account sounds good?
( 6m 48s ) Francesco: Yes
( 7m 16s ) Sunita: Please do check it has been added, you can link the account by referring this link, apart from this anything else, I may help you with? https://help.ea.com/help/account/link-your-console-accounts/#how-to
( 9m 8s ) Francesco: Oh my god it worked, thanks a lot!
Chat Ended by: Visitor
Chat Duration: 9m 15s
Only note, I was on a US VPN using a fresh new installation of Firefox. It may actually work without all this setup, but anyway I just did it this way.