Yes, it's possible to revert to an original/older stock CSC while the PDA and Phone parts are of a later release date. The best thing to do may be to look for the latest CSC for your country, then flash it into your phone using OCTANS. Updating the CSC will wipe out your phone's contents, so do backup your stuff before you do any flashing.
I encountered this same problem that you're having, and I know how disconcerting it can be to have your keyboard stuffed up...
Ignition, on Jun 1 2010, 08:09, said:
So now that i upgraded to this stock rom, i installed both the PDA and CSC......
my keyboard is all wrong (not all buttons map to correct symbols) so I wish to now flash back to the original CSC... is this possible ?? to now just flash the CSC part to an older version ??
i am assuming you can mix and match different CSC and PDA versions of the stock firmware ?