About 95% sure the K702 pads won't fit the K550. The K702 pads rotate and click into place with small plastic ledges or clips. It looks like the K550 doesn't have those based on the picture.
How are the K550 pads held into place? I remember taking them off once but now I totally forget!
K550 pads on the K702 might be..interesting
Actually when I pulled the pad off, one portion was hanging on to exactly what you just mentioned - a small cutout ledge. I applied more pressure to remove the pad. It actually is there in the photo, second group of pictures, the one in the bottom right. It's at the 10 o'clock position.
So the pads just might be interchangeable.