We need the manufacturer and device first. Yes, Microchip does provide Flash to EEPROM implementation. And it all depends on the device. If using flashed based PIC that does not have EEPROM, there are 2 types. Type 1 lets you write to flash at run-time and lets you write to individual random access bytes. Type 2 lets you write to flash at run-time but you must first erase a huge chunk of flash first, and then write to flash. Since a huge chunk is erases this of course raises the flag of "must I first save the chunk". That would be a yes if you want to keep that data and you must be all powerfull enough to know what chunks to access else you will suicide your own code at run-time.