Have a cleanout valve that need to be extended. Need to know how to femove the fitting without cutting it off.

You aren't going to have much luck without cutting. The glue used with PVC is actually called a weld for a reason. Any chance of a drawing or pic? If you shop at a home center you should find couplings that can be installed in the existing clean out instead of the cap. Not sure about your layout but this should extend the cleanout as far as you want.