XEP's aren't downgradable, it's in big ugly bold letters everywhere in the firmware, but I've never had an issue with any of the later XEP firmwares.
In the case of the Navigator and the M1, a lot has to do with the products basically being re-engineered/branded Ness products. What comes out overseas takes a while to get through UL and from what I remember hearing, Elk doesn't put a huge effort in remaining compliant or up to date (look at their list of supported 2 wire detectors and their explanation on 4 wire units...compared to their contemparies).