No, I DID mean ONE PAD at a time. Remove ONE pad, leave the other pad in that caliper in place, push the piston back into the bore and put the new pad in that slot. Then, remove the other pad in the SAME caliper, push THAT piston back into its bore, then put in the SECOND new pad in that caliper.
If you remove them both and push one piston into its bore, you risk pushing the other piston too far out of its bore. If this happens, the piston is beyond the seal and it will not go back in without disassembling the caliper.
I would NOT open the bleed screw when doing this UNLESS you have a pressure bleeder or a helper to properly BLEED the brakes afterwards. If you leave it closed, do one PAD at a time, then you can thoroughly flush afterwards. Flushing is much easier and more trouble free than BLEEDING air out of the system.
There are TWO lessons here; one PAD at a time and FLUSH YOUR BRAKES ANNUALLY. If you are flushing annually, there will be no ill effect from pushing fluid back through the lines. Thoroughly flushing annually will almost certainly prevent the need for ANY brake hydraulic repair during the life of the car.
Best of luck,