i dont like it when i discover, say rifling, a few turn before the AI, then try to build up some sort of army to attack the AI, then a few turn later they discover rifling and suddenly ALL their defense units have gone from musketmen to riflemen, IN ONE TURN! I really dont enjoy that, because if it wa the otehr way around, and the AI wanted to attack me, i would stand no chance. so although the AI is pretty stupid, this advantage to the AI is way overpowered. so i think they need to balance it more. perhaps make it cheaper to upgrade ur veteran units that have more promotions, so that it is more worthwhile to keep them. or even just reduce the cost of all upgradig by 50-100 gold. its ridiculous that i cant build a rifleman in 2 turns, yet it costs me like 200 gold to upgrade him (which would prolly take like 20 turns to get that much gold).
so yea, im not a big fan of this little detail of the game!
take it wasy dudes.