I switched carriers on a C RTW on the route GOT-FRA-SFO from LH to SK/UA combo so I could get my UA 100 % bonus miles. As I remember the nice SK agent in GOT told me that I wouldn't have to pay the $75 because I used the exact same routing. If I switched to GOT-MUC-SFO then I would have to pay a reroute fee. So, at least as of this summer, switching carriers on the exact same routing should not cause you to be charged the $75.