One can replace the basepads on the stock magazine, however the pistol, (with magazine installed) must fit the IDPA box AND the basepad can not be a weighted base pad (i.e. basepad under 1 oz).
Robert Ray also made a clarification last year, that basepads can be changed out for the SSP division as long as the pads are plastic or aluminum and do not add more than 1oz. to the stock magazine.