I agree with Chagrin. If you have a normal MEGA2560 board, it uses an Atmel 16u2 or 8u2 for the USB interface, and you'll need to reprogram that with different firmware if you want to change the VID/PID. There should be tutorials for the reprogramming part "around"; that's also the method by which the board is reprogrammed to be something other than a serial port.