[EvilTerran]

When a user whose password is hashed the old way logs in, after checking the password they just supplied against your stored old hash but before forgetting the plaintext, you can compute the new hash & update your records.

Of course, that only works for active users - it won't upgrade anyone that never logs in. Depending on just how weak the old hash is, you may want to eventually cut off any lingering un-upgraded accounts: just forget their old hash, requiring them to go through your password reset process should they ever come back. If you've left it long enough, those accounts will probably never be used again anyway, so that should be NBD.