There will be an option soon to unlink the swtor account see https://help.ea.com/en-us/help/star-wars/star-wars-the-old-republic/swtor-on-steam/
I had the same problem. I've not played in years but felt like returning. I have two accounts from years and years ago. The first one I tried was just an old low level character from when I was testing the game. I found my old email for my cool high level characters and tried to log in. So now I'm locked out; "We can't log you in. Please double-check you're using the Steam account you usually play on and try again" and stuck using the low level account.
I am not downloading 200+GB again to get the classic launcher. I would have paid for a month of subscription to play my old high level characters but not to start again with low characters. Way to fail importing your games to Steam EA.
