Back to the original topic of the thread I think Taiwan should go both routes. Upgrade their A/B birds with the -229 engine, AESA radar and whatever else they can get their hands on in addition to acquiring new Block 52+ aircraft to ensure the most commonality across the entire Viper fleet. Since the A/B aircraft are already to MLU standards I would guess subsequent upgrades could be accomplished at unit level speeding up the modification process. Upgrades can be kept classified if need be. Purchasing new C/D airframes would allow older aircraft to stand down in be upgraded without a huge dent in readiness levels. I'm sure the USAF and Taiwan can figure out a way to get new Vipers in country without letting China know. I know the mainland has operatives/spies on the island and probably in RoCAF , but anyone will be hard pressed to notice the difference between the blocks. Especially while in flight and/or at a distance.