Wow, that is a bit weird (or maybe wired). Didn't see all the circuit boards in my Rugged Radios headset, but I didn't dig into it very far, either.
Despite the complexity, it's just the speakers that need swapping. Disconnect the wires to the 300-ohm speakers, solder them to the 8-ohm models.
However, gotta ask...it's not an ANL unit, is it? Not sure what a speaker switch would do to an ANL unit...nothing good, I'm sure.
I now have ~10 hours on my converted Rugged Radios headset, and it's been working great. No problem at all, volume-wise.
Ron Wanttaja