Mass change usergroup if member has been in it long enough

[MagicPID]

To make a long story short, I am sorting out over two hundred members from a paid "Premium" usergroup. Another administrator started accepting payments via a system that wasn't supported by the Paid Subscriptions module at the time, so we ended up foolishly keeping an Excel table of when we needed to move a user back to the regular usergroup when the subscription expired.

It sounds foolish, and believe me, it was. We've lost that Excel table now, or rather, we've lost track of everything in it. We now use the Paid Subscription module with PayPal, so a good handful of users don't have to be worried about. However, for the rest of them, I'm completely unsure what to do. We're losing quite a bit of income as I know for a fact there are people that should be expired by now.

Other than doing this by hand, is there anything that can be done to quickly sort them? Does vBulletin record the date they were changed to that usergroup?

Thanks for your help.