Yes, all of the old threads are coming with us!!
This includes all of your PMs. You will now have the ability to download all of your PMs too, which helps if you want a history to save or what-not.
Yes, we will all be here helping people learn about the new software and helping people adjust. It will be quite the anxious experience, I know... but we'll do what we can to help ease the transition and answer people's questions after the move.
DocJohn