I have a 30amp duplex breaker (220) for my old A/C unit. My new A/C unit only requires a 15amp duplex breaker (220). Can I keep the 30 amp breaker or do I have to change to 15 amp. I know that I have to change the receptacle to a 6-15P from the existing 6-30R

You must follow the installation instructions for the equipment..
If you don't you likely would void the warranty.

Providing a fused disconnect "at the unit" with 15 ampere fuses would be a good solution, then you can keep the wire, assuming it is #10 awg, and the 30 ampere circuit breaker.

Once you change the receptacle to the 6-15R, the breaker needs to be changed to a 15 Amp.