To my knowledge, there isn't a converter for reversing the process. Your best option is to compare the table structures and see if they are easily restored to CPG-BB. Even so, I think they also play with forum permissions, then there is the wordlist and match tables to rebuild, and they also mess around with the cms users table.
The forumspro conversion process is not very user friendly and it interferes with core tables, so it's a lot of work for anyone to do, and carries a great risk of screwing it up.
If you are not comfortable with that, then just stay with forumspro and run with minimal features to reduce the load.
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):