Need SEO advice merging 2 forums and moving to a new url

I run two car forums that are model specific.

One of the forums is installed on the root and the other is installed in a forum category "/forum/"

carmodel1.com (300 members_

carmodel2.com/forum/ (300 members)

Im going to merge both those forums together and put them on a new domain thats more brandable.

I really dont know where to start on how to forward my traffic to the new url and what i should do on the old sites.

Any advice is appreciated.

The only idea coming to my mind is to contact web developers since you are about to move forum databases with information of posts and members and merge them into one and then launch a new project.