Swapping pads question

[Professor Peach]

Hi folks - the time has come to swap the pads on the D2 S8.
I'm doing all (front and rear) as they're all fairly well worn. My question is this. As I retract the pistons, presumably the fluid level in the reservoir will rise. How much will it rise? (I obviously don't want it to come over the top). Is this likeley to cause any air in the system to require bleeding, or should it be very straight forward?
Perhaps I should stagger the process and just do the front now and the rear much later (as they aren't worn anything like as much).

Cheers

Rob