Even if the 4NF was grandfathered in, it would only be for a period of time. I'd expect them to eventually 'realign' the benefits on the legacy cards with whatever the new offering was, especially if that's the reason for the whole reboot to begin with. It might take them a while, as people's renewals come up. Perhaps with Dodd-Frank & CPFB they are less able to make radical changes to the Terms & Conditions for existing holders.
I would observe for all the comparisons to Amex Plat, that the Citi flight $250 credit is super easy to use and way more flexible. No messing around with gift cards, picking an airline, etc.