The major part is the torso. Because of the way it's constructed you must use the cockpit housing of the Strike. Technically the difference between the two is slight but significant. I've highlighted the differences here in red. Thanks to Dalong for the scans.
Because the cockpit housing is built as one piece you have to use the entire unit, even tough 90% of the construction is the same. The re-occuring theme with this is that the PG Red Frame actually uses less parts than the Strike. At least concerning the frame.
After swapping the parts it looks like this. The neck and shoulder joints are slightly different. The torso's outer frame is significantly different than the Red Frame.
Couple of slight differences concerning the arms. Just the elbow/forearm armour mounting and the shield attachment unit.
I did repaint the foot of the Strike black because the red just clashed too much with the colour of the frame. I like it better like this. Actually, the Red Frame comes with almost all the parts necessary to make a complete Strike frame but there were just counted as extra/useless parts on the runners in the Red Frame manual. For example the Red Frame comes with the Strikes "Combat Knives" but they were considered 'useless' parts for the Red Frame.
Conclusion - Not a lot of differences in the frames at all. Bandai certainly saved a lot of money in this area. Frame wise, the Red Frame uses about 90-95% of the Strike frame but the Strike uses 85-90% of the Red Frame. Consistent with SEED chronology, the Strike frame clearly came first in the development process and Red Frame is just a slightly slimmed down version of it with a couple modifications here and there.
It's funny that despite the percieved bulkyness of the Red Frame with armour, it actually has lighter and thinner frame than the Strike. However the Strike's armour is considerably thinner and less bulky than the Red Frame. Perhaps this is to better accomodate the various Striker packs it uses.
More frame comparisons to come but next time we'll look at making a Red Frame from a Strike frame.