I'm not sure about the Simple Subforums mod, but I upgraded from a Categories Hierarchy modded board. You'll need to use phpmyadmin first to ensure that each forum is assigned to a Category. Alternately, you could also un-nest all of your subforums into regular forums from the ACP in phpBB2. The main problem with CH is that it removes the use of category and makes all into forums, so editing the database is a necessary step. Before conversion, I'd suggest installing a vanilla phpBB2.0.22 unmodded to make sure it works properly. It might be extra steps, but it might be worth the trouble.
After upgrade you should edit each forum in the ACP to re-nest them back to the way you want them.