When you try to login thru FB, it says clearly that you need to enter a NEW Email/Pwd combination to login to that old FB-based account.
The form is very, very clear. See screenshot.
So...
more... once again in detail:
1. Every account has its OWN Email/pwd. You guys already have MULTIPLE accounts in IRI with many different email/pwd combinations.
2. Previously you could also login thru Facebook directly. Not anymore.
3. SO, just provide ANOTHER NEW EMail/Pwd to login to that old FB-based account.
How to fill up the form:
1. Fill NEW email.
2. Fill NEW pwd.
3. Click the button.
If the email is already used in some other account, pick ANOTHER Email.