I'm hoping someone can point me in the right direction with this problem...

I'm trying to modify a large forum with thousands of users, and would like to transition to a subscription-based setup. I need to keep all the existing posts as viewable, but require users to re-register with certain required info, and subscribe before they can post again. I also need users to be able to keep thier existing usernames and passwords after subscribing, and prevent new users from registering under exisiting names...

Is there a simple way to go about this? Would it reuire sending an email to each user asking them to reregister?

Any ideas would be helpful...