Expanding on the system power supply voltage issues, as the supply rail is changed to 3.3V be sure to check that the input voltage thresholds, the output voltage levels and the analog voltage ranges are suitable for the components in the system that the dsPIC33 device will interface to. If the entire system is designed to operate at the 3.3V supply rail required by the dsPIC33, this process will be simple. If the system has components that must run on a 5V rail, then interface issues will be more complex.