Any way to do this without completely losing my search engine ranking? redirects maybe?
I started my forum 5 years ago and had to settle for a 2nd choice, the one I really wanted expired recently and I managed to snatch it up before the resellers got to it
I think you've already answered your own question.
You will need (dynamic) redirects so olddomain/thread/1 will automatically redirect to newdomain/thread/1