If you use the plastic figures from Tamiya, Italeri, etc., you might want to look into getting a hot-knife to make the changes.

I have one made by X-acto. I think it's item # 378. At least, that's what's stamped on the face of it.

It has a soldering tip which can be removed and replaced with the standard set of X-acto knife blades.

When hot, you can carve regular styrene plastic almost like butter and it's a great help in making such alterations. With it, I've been able to transform even such things as an Airfix redcoat with a brown bess musket into an 18th century gentleman with a double-barreled shotgun.

It has the added advantage that you can undercut many of the clothing features. For instance, cutting under the wrists of jackets so that you can see partway up the forearm, cutting behind lapels to give the appearance of the shirt going inside the jacket instead of looking like just another molded piece.

Or, I've taken small, 1 piece solid parts like 1:200 scale 37mmFLAK and cut them apart so that I could add hinges and make them traverse/elevate, etc.