Perhaps in hindsight, you guys should have rolled out a soft-launch for registrations, enticing LFG users to register for an account and link their gamer profiles in order to have a special format for their posts, indicating it being from a "verified" member?
Then everyone would have been scrambling to find out, "how do I get that cool verified status on my posts?"
Additionally, registering for an account could have given them access to the beta site to help with feedback and bug finding.
You could even have updated the LFG site to include posts from the FTW database in its listings. Registered users' posts could have been put onto the FTW database instead of the LFG database and included in a combined list on LFG.
Slowly, posts would be moving over to the FTW site behind the scenes, but everyone would still be seeing them in the LFG interface.
Once you were down to stragglers left on LFG, you could start making it obvious that they were going to need to register soon.
After that, the only change they would be faced with when FTW went live, was a new look-n-feel.
Instead, flipping the switch one day and forcing them to register before posting the next day, rubbed a lot of people the wrong way and created infrastructure related nightmares (i.e. SPAM detection).
I know this is all in hindsight, but having dealt with significant changes to systems serving high volumes of users in the past myself, I could have predicted this issue had I known ahead of time what was planned.
Feel free to hit me up in the future if you're thinking about implementing significant changes...