I use razor saws from X-Acto. I don't recommend using a knife; you'll have better control with a saw.
I agree with Simon that you'll wind up removing some detail in the cutting and converting work. But you can use putties to replace the detail and sculpt new--cartridge pouches, knapsacks, belts, but also limbs, in short, any detail you want to use. There are a number of choices for putty. I use Aves Apoxie Sculpt, and Miliput, which are both 2-part epoxy putties. You mix equal parts of the putty and a catalyst, kneading them till you have a consistent mix, and then sculpt. Both of those putties are air-drying and curing putties. I also use Sculpey products, which do not need to be mixed, but need to be heated to cure. The instructions direct you to bake the putty in the oven at about 110 degrees F, but I know some guys who use a hair dryer to cure it, especially when used with plastics or resins, which don't hold up too well to baking.
There are other putties, too, like Green Stuff, but I don't use those. Others here in the forum do, I think, and they'll be able to speak to those.
Do show us your work!
Prost!
Brad