I'm largely fine with the gates being reprogrammed, so you have to be let through manually (even though ti is a pain in the arse), if it's the case that a lot of folk were using the tickets to other destinations. However, the gateline staff need to know the validity of the tickets; I am still seeing people tweeting GNRailUK about not being let through the gates even when explaining they are going to Moorgate.
Wouldn't an easier solution to be for tickets to go through at King's Cross and be rejected at gates at destination stations, except Old Street and Moorgate (where they would let you through)?