Now don't take this as 100% accurate great gospel, but I believe any techs that will improve stats would have the opposite effect: 'current' units won't benefit until a model upgade, while any new units will have it. I got this result in HoI2ArmA after experimenting with few things and with my minimod with the following test: I started game as Brazil, look at my naval units, then researched Sealane Interdiction (as it increases range in my mod). Those naval units were unaffected, but any new ship I was building had increased range on their stats and also when they were ready to be deployed. Same also applied if I E.G. started a game with a country that has no real landdoctrines and then went for superior firepower (+1 SA).