Hi there,
In order to unlink a platform account from your EA account, you must have access to the EA account. With that in mind, the solution here would be to recover your old EA account and then unlink your Steam account. You can find instructions on how to recover your account here: What to do if your EA Account has been hacked.
Once you've recovered your account, you can then unlink and relink to a new EA account: Unlink your platform accounts from your EA Account & Linking your platform accounts to your EA Account.
If you're having any problems making these changes or recovering your account, please feel free to contact EA Help Support for further assistance:
How to get help with your EA Account