I guess one point no one has mentioned is whether or not they moved the saddle up or down when they swapped cranks. I firmly believe that the saddle needs to be set in relation to the pedal at its lowest point. To set my seat height, I have always used the classic technique:
1. set crank to be in line with the seat tube
2. sit on bike in riding shorts and stocking feet
3. there should be a slight break in the knee with the heel on the low pedal
4. when the saddle is at the correct height, you should just be able to spin the cranks backwards with your heels on the pedals, rocking only slightly
My saddle height using this technique has been corroborated by more sophisticated methods of determining proper saddle height. But the point of my post is this: when I am using 170mm cranks, my saddle is set so that it is 79.0 cm from the middle of the bottom bracket. With 165mm cranks, it goes up to 79.5 cm; with 175's, the saddle goes down to 78.5. Everything is in relation to the pedal at its lowest position. If you keep the saddle at the same height, I think you can wreck your knees with the wrong crank length.
L.