When I did my swap last year, I bought the new set of mirrors for the 2nd gen from wyatt earp. I bought the 3rd gens from private sellers on ebay. I also bought another 2nd gen from a private party on ebay. Of the three from private sellers, two I had knowingly purchased damaged on the back of the mirror heads themselves.
When I did the swap, I was able to remove the mirror motor and mirror from a 3rd gen damaged power head and then screw it right into a 2nd gen mirror head after installing that head onto a 2nd gen arm. The 2nd gen head that I put it into was older than the ones I bought from wyatt earp.
One the other side, I changed it at the pivot where the arm folds back. My point is that on the ones that I had, I didn't just use the hinge where the arms fold back, I also used the pivot where the mirrors folded out to make my swap. I don't know about the newer ones, but take a look, you may be able to do it there.