When the new forum released, I migrated all of the old users over after about two days of it being live. The accounts that I do know the passwords for appear to all work fine, but I wouldn't be surprised if some accounts are stuck in some sort of limbo. I haven't run into that yet, but the new forum is a completely new website so it's probably something I should've expected earlier.
If you had an account on the old forum and you're unable to login, please email or speak to me in IRC and I'll invalidate the screenname status your the old account(s) for you so you can re-register (if you're unable to register under your original name of course).