If you loosen the bleeder valve on the caliper, you can push the pistons far enough to get the new pads in. Get a small hose to insert into the bleeder valve and drain the excess into a receptacle of some sort, or you will have to clean up all of the spilled brake fluid all over the garage floor.
Don't forget to top off the reservoir afterwards.
Removing the rear calipers is pretty easy (done that accidentally not knowing how to change the pads), but can't say what is necessary to remove the rotors...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car