I would like to replace the old style shut off valve with the new 1/4 turn angle valve. My question is can I just unscrew the red part and screw in a new 1/4 turn one to the blue part so I don't have to make cuts to the pex pipe?
This doesn't appear to be PEX pipe but a 1/2' or 3/8" compression inlet. Remove the valve by unscrewing the red part and take it to your plumbing supply store so they can attempt to match the compression face and threads. If the compression sleeve and nut come off the pipe easily, take them with you too. Your valve stem is PVC so I'm curious as to why you want to replace the valve.