I bought cheaper replacement pads on Amazon that did not fit and did not snap into place on my headsets. Reviews complained about them falling out, and they certainly did.
I decided to buy these replacement pads directly from Sennheiser and they snapped into place and work perfectly on my HD 650s with no modifications. My headphones are like brand new again! If they last as long as the previous ones I'll get another 6 years out of them, and since I wear these headsets every day, $50 for this much extra life is a bargain.