We don't actually know what changes Canon makes between fw versions (although comparing disassemblies from fw dumps could reveal this if enough time were put into it). I find it completely possible that you have two cameras with different hardware, requiring slightly different firmware. I don't know how likely this is, but they could for instance use a part from two alternative vendors (choosing the one that's cheaper and available at a given time) that provide the same functionality but are for example initialized using different commands on a serial bus.
If that were the case, forcing a firmware update would not be a good idea. As for how to do it, I guess you'd have to hack (decrypt, disassemble, analyze, modify, triple check, encrypt, run, hope for the best, buy another camera to replace the one you've just bricked
) that particular fw update to bypass this check.
