I did quick search on SMF forum, but couldn't find information about multiple subdomain for each forum section/category.
For the old domain you can add a 301 permanent redirect to the new domain, search engine's will see the permanent redirect and update all your search results, it should have a very minimal impact on your search engine rankings.
So if someone goes to http://www.olddomain.com/forum/thread/1/ they would be redirected to http://www.newdomain.com/forum/thread/1/
Search engines are intelligent enough these days to handle these type of instructions.
Best
The bold area shows it could be possible for subdomain too but it will require the admin to do a whole lot of job to fix it.