Back when release 69 added the ability to convert identity accounts to full accounts, a few of us tested to see what happens if you continue to use the identity login (Facebook, Twitter, or OpenID). It turns out that logging in that way will log you into the full, regular, LJ account. This is actually a really neat feature and it should probably be mentioned somewhere in FAQ 314. Maybe something like this added to the end.
Logging into a converted account Once you upgrade to a full LiveJournal account you can continue to use your Facebook, Twitter, or OpenID account to sign in, foregoing the use of the LiveJournal username and password. Simply visit the login page you used previously and fill it in as you did before the account conversion. You will be now be logged into your full LiveJournal account.