It's actually been done before by another member on the boards (and looks pretty spiffy):
TFA helmet on Strike sculpt
That's a great mod, look good. The only thing I notice is that the jaw seems a little smaller/shorter than stock strike head, still ok.
The other mod in the same page which shows TA face in TFA helmet is also very nice. Maybe that TA face can fit in GA helmet well, TA and GA head are both smaller.