Considering the level of fly-by-wire that goes into airframes nowadays, I could certainly see such features added to older airframes through flight-software updates after they are already in service. I have no idea if that's how it's done in the field, though.
Software updates are done. However the manufacture has to recertify the airplane with the FAA first. This is expensive, requiring months of testing and a lot of paperwork. Each plane model getting the update needs to be re-certified, it isn't good enough to say that it works on this other model even if the planes are identical for purposes of that software.
