There are two ways to unlink a SXSW credential from your SXSW Account. After unlinking, you can log in to social.sxsw.com with your preferred SXSW account and apply the link code you just freed up by unlinking.
Via SXSW Social
Click the “Unlink this Account” link at the top of the page.
Via the SXSW Schedule
Hover your mouse over your name on the top right of the screen, then select “Unlink A Badge”.