I can't speak on the wraithguard but have recently been playing around with enlarging my oldschool plastic terminators.
I sawed one in half at the waist then split the legs to pose them a little wider. Spaced them with some procreate and then gave them a robe overtop to hide the seams and blend them with my Deathwing.
I think it came out halfway decent but wasn't worth the time invested. The rest of them are getting bases that are built up a bit to bring them to the same height.
I don't think I can get clean enough cuts with my razor saw to try it on the metal guys (that and they look like lead) so I'd rather have them short and detailed then bulked up and obscured details.
Edit: The new plastic arms look a little out of place on the old terminators. Just compare the assault cannons for an idea why.
Another Edit: Decided pictures speak better than words. Here's the arm comparison. All apologies for the blurry cell phone pic but it gets the point across.