RDS Radio and Code for lock out

I got a malibu with a RDS radio. I recently bought a stock gm cd radio with rds. Well, the cd player is saying check cd. Anyway to fix this short of a dealer to do it. OR if I put my old RDS radio with my cassete, it will say locked once I start teh car up. Can I get the code before I switch the radio. OR is the code only available when the radio is locked? Or is there a code availible. Any help will be appreciated.