The 700 model CHM will fit into the 500 model Roombas. The entry of debris into the 700 style CHM still occurs just at a slower rate. Here is a URL to the new procedure to ball bearing modify the 700 CHM:http://www.robot-doc.com/21.html
One other point that is overlooked but happens on all Roomba models is the wearing down of the brush gear shaft diameter and also the enlargement of the opening the gears spin in. In the past brass and copper bushings were used to slow the process but if ball bearings are installed that process is halted.