What you need to do is take your forums offline.
tarball up your ubbcgi and noncgi directories
eg. Run this in your ubbcgi directory
tar -cvzf ubbcgi.tar.gz *
then do the same in your noncgi dir
tar -cvzf noncgi.tar.gz *
using tar will keep the file permissions intact.
FTP the files to the new server, create your ubbcgi and ubb dir's chmod them to 777. Then untar the files into the approproate dir's
tar -xvzf ubbcgi.tar.gz
and the same for the other file.
Then go into your cp and update all the paths.
Rebuild threads and you should be done.
I've done this too many times to mention