"And I'm sure just cause CAA/NATS approve something everyone will rush out and upgrade right away right? I doubt it. "
Depends what is required to "upgrade". If you have ES (supported by just about all Mode S transponders except Garmin as standard) then the upgrade is an RS232 cable. If you have Garmin then you are right, not many will scrap a new 328 to upgrade to an ES unit costing about the same (or less) as the 328 did. Personally, I already have the cable in place but have the com port disabled on my EFIS.
Rod1