Hello ... those that have the letter A, are the ones that will end up replacing those without A. That does not mean that both do not work well, but that it is the same a little more improved in terms of electronic characteristics. They are cheaper, have FLASH memory (hence the 12Fxxx) that is faster, and can support frequencies of 20mhz while others do not work at 4mhz. If you are going to work at a clock frequency of 4mhz you can use both interchangeably, but if it is higher you must use the 12f509.With the .hex file you have no problems, unless the program is configured to run at frequencies greater than 4mhz . Regards