On the one hand, that sounds easy, as the prop guns fired blanks anyway, and I believe you can see shell cases coming out every once in a while.
On the other hand, it means you have to digitally erase every laser blast in the film, AND add in digital blood and gore every time someone gets hit.
As someone who generally disapproves of a lot of the ideas in this thread, I'm surprised to say that I really like the idea of adding mechanical movement sounds to Vader. It does make sense.