Well, wouldn't it just be a matter of slightly changing the animation?
I'm probably oversimplifying but that kind of seems all that'd be necessary. I don't know anything about programming though, so it could be any factor that they have, or they just wanted to do it for shits and giggles.
I don't honestly know, that's why I call it wizardry. If I had to hazard a guess, I would say they could
a) mokap (or at least manually animate) two versions of every move, but that means storing two sets of animation in memory for every character AND tweaking two sets of animation for any buff / nerf / cosmetic change down the line.
b) They could build models that are uniform enough to accept the same animation from either limb (and build an engine that supports it), but NRS likes to get pretty creative with their models and animations, so it may not be feasable down the chain.
Given what I know of capcom's dev processes, I'm assuming they're using that second one because they could give a shit about creative modeling. Even so, there's always the minute chance that UE3 simply can't or won't do it. Capcom uses Havok and a couple other physics engines for their environments, but I think MT Framework (which powers Marvel / RE) and whatever makes SF4 / SFxT run were totally built in house.