Twisting the pistons in (to make room for the new pads) pushes brake fluid back towards the reservoir. So maybe it'll overflow. Depends on how full it was before you started.

You'll want to flush brakes, to get fresh fluid in all the lines every 2 years or so. Lots of people like to do that when they replace pads, just because it's a good reminder. Or just an excuse to do both jobs together.