for ROM read ROM/PROM/EPROM/EEPROM/FLASH, depending on what you have

1) did you try loading the old program into the ROM ?
2) how did you verify that the .HEX file was identical ?
3) did you try to update a unit working with the old ROM
4) separate ROM or ROM in uC
5)what is, what was the uC
6) what is, what was the ROM

Erik Malund