Nevelon wrote:
I’ve not done a head swap on that model.

One thing to consider is how much of a “swap” do you want. If you just want a new head on a model, and don’t care what happens to the old one, heads are pretty simple to work. Just clip, cut, and carve until the new head fits.

If you want to salvage the old one, things get a lot harder.

 SmallChanges wrote:
The techmarine which comes with servitors is pretty flat around the helmet area, so you might even be able to salvage the original one if you're very careful. Finecast cuts pretty easily with a standard craft knife, but watch your fingers (a rocking motion along the length of the blade will work more safely than strong pressure on the blade).

If it's the one with a bare/bionic head, I doubt that head can be salvaged. You'd need to hollow out where his head is to preserve that raised collar, which may or may not be big enough for a helmet. If not, you could trim the sides of the collar down level with the front, which may give enough space for a helmet. Don't forget to hollow out a space for the neck (a 3mm drill or some very careful dremmel work would do well).