All existing OpenID associations should be intact and you should still be able to access the associated Bitbucket accounts through the login form. Our new OAuth integration with Google unfortunately limits users to one account per service for the time being. We may look at expanding this to allow multiple connections in the future, but there are no plans to do this in the near-term.
Just got bitten by this. A workaround is to use authentication via Google for one Google account, and user/pass authentication for the other Google accounts - just need to remember which one works and which ones take you to the "Error 500" page.