And conquistadores keep all their special promotions
What
Maximo the Xth said and also correct me if im wrong, but I dont belive that there ever was a time when cavalry/helicopter could build cities after they were upgraded from conquistador.
Most UUs with special promotions are reasonably well balanced between them
Now that I have realized that some civs UU’s may actually keep their special abilities trough out the game and some other civs may not, I would strongly disagree with what you are saying.
The worst cases are Rome and Greece, yet with the right conditions they can do quite a lot of damage in the early game.
Well they can surely do some nice damage
IF in war in that particular moment. Still its way off balance when compared to civs that can have nice bonuses trough out the game after upgrading their UU to something else. Also in vanilla you can pretty safely add at least England to that “worst cases” list of yours.
The advantage for England is not that huge in that regard, it's just a huge advantage for longbowman right off the gate.
Well first of all im not just talking about the Englands UU’s, im talking about every civs UU’s. Im basicly against this loophole because for the sake of AI, as a player I do not want to have even the possibility to have this kind of advantage over the AI. Of course its “cool” to be able to build something like 5 longbowmen and then just without war be able to upgrade them to gatlings/MG’s with +1 extra range. That actually is pretty cool, im not disagreeing with that, but for the sake of balance and more over, for the sake of the AI its just simply not a good idea. I do realize that there are many aspects in the game that without them the AI would actually perform better when compared to human player, one thing that crosses my mind right away is the 1upt, infinite stacking would be a lot easier for the AI to handle. But unlike the 1upt this thing with these UU’s is, –as far as I see it- completely unnecessary and brings so little to the actual gameplay, but on the other hand, it can be quite a big loophole against the AI.
Stripping that promotion on upgrade would actually punish England
Just do the same for everyone else. This way if some civ actually has more strong UU than some other civ, then they would only be able to use it in certain period of time and not like half of the game, like I belive is the whole original point of these UU's. It also helps the AI because it cannot use the potential of these units as well as human players can (its basicly a loophole), so limiting the time of these UU's abilities to just that particular unit actually helps the AI.