The Toyota/Honda comparison is a bit over blown, but the commonality between the two frames (or lack of it) means you will probably need a frame specific
headset, and maybe a fork, depending on whether the fork steerer tube is long enough for the longer head tube of the Soma. You will need new long reach
brakes and ?? seatpost, as the specs for the Giant frames do not mention seat tube diameter. The bar, brifters, BB and chainwheels, wheels and deraillers all
will swap over (along with the stem perhaps). Then the question arises of what to do with the stripped Giant frame. The Soma frame is really a touring frame.