The PCM itself in isolation doesn't know that it's been changed, however other modules do. The theory on this is the BCM has copies of all California CARB compliant checksums. Once the car is started with a PCM that broadcasts a different checksum the BCM says to the PCM "hey bro flip 1400 on for me" and the PCM complies.
Also another theory, the PCM says hey, BCM, what checksum should I be on? BCM says bro "ABC". PCM says well I'm on "XYZ" lemme alert someone with a 1400. This seems to be the more plausible event because the Demon owners that have put in the other PCM get a 1400. Anyway, I have a BCM from ebay I'll be ripping the firmware soon and can confirm.