If you can set permissions successfully the DB is functioning correctly and therefore the common.asp in the Forum/Admin folder has been correctly set with path and driver. If you get errors when using the Forums, it suggests that the other common.asp in Forum has been incorrectly set.
Both common.asp files require configuring with the path to your new DB and the appropriate driver.
Also, what do you mean by switching back to your old DB. v6 DB requires v6 software and v7 DB requires v7 software - They are not interchangeable. My apologies if that is stating the obvious.
Edited by Nigelo