You can fit a radio or transponder of the same part number if one becomes unserviceable.
You can't fit another type of mode A / C or 760 radio.
No longer true, this possibility was discussed quite lengthy when EU-1079/2012 got valid and denied by the authorities. Starting November 17th 2013 every radio to come into service has to be 833 and every XPDR put in service has to be Mode-S. There are some windy avionics shops telling you another, but we had already two plane immediately grounded at ramp check due to that.