Based on my past experiences, you can read a radio with your current version and the lower numbered ones, but the higher one has to be read with the same version number that programmed the radio or a newer one. So in your case, a R18 won't be able to read the programming from an R19 and higher. But if you have a R20 version, you can read all of the lower numbered ones. You can always write to a radio that has been programmed by Programmer with any version, but you may not get all of the benefits of the latest updates that may be available from the software. However, once a radio has been read or programmed by RPM (Radio Personality Manager), then all bets are off, and you'll need to follow the downgrade process in the first thread.