I am not trying to argue for or against unit stacking with this post(though I do favor 1upT.) I am merely trying to understand this idea that it'll be practical to simply "mod out" 1upT. I am not sure the people saying this are really thinking this through to the end.
1) Units will be balanced for 1upT. This is a non-trivial point. Every aspect of every unit will have to be rebalanced for any stacking mod.
2) Ranged combat will be based on 1upT. There probably won't even *be* a first-strike ability anymore due to the change in the system. All archery units will have to be heavily re-conceptualized to account for stacking. Everyone can understand why it will be problematic to have a stack of dozens of Civ5 Archers.
3) The graphics code may not be set up for stacking. In Civ4 the engine makes a basic assumption and has certain behaviors when units are stacked. There isn't necessarily a behavior for this in the Civ5 engine.
4) The entirety of the city defense concept in the game is based upon 1upT. You can't simply change the game to stacked units without radically rebalancing city defenses.
5) The streamlined fleet-transport system we've heard about is likely 1upT centric. This may be a very difficult change if the game isn't already designed to support ship units flagged as transports.
6) The AI will be based around 1upT. This is perhaps the most intractable problem for any "fix" mod.
I would argue that, more than likely, you aren't talking about a "mod" at all. You are talking about a total-conversion or perhaps a new game. Anyone inclined to "add stacking" to Civ5 might be just as well off adding hexes to Civ4! I'd recommend that anyone who just can't accept 1upT stick to Civ4 modding.
