From the Futaba 7C FAQ...

How do I change this radio from FM to PCM or vice versa?

This radio comes with the ability to transmit in either FM or PCM modulation. Simply change the modulation type in the parameters (PARA) menu, and turn the transmitter off and back on to transmit in the new modulation. Of course, the receiver must be the proper modulation to operate. A receiver cannot be converted from FM to PCM or back