It happens most of the time with Avid’s, Vicky. If folk changed their pads sooner than going right down to the backing plate it would make things a little easier.Half worn pads can often be removed and a clean up and lube of the pistons can be enough to enable new pad fitting without any other work.
More often than not you will have to clean the pistons and lube them. Reset them flush into the bores and then bleed them for the best outcome.
Shimano’s are no problem at all. You remove the pads, clean up the edges of the pistons and lube.. then they’ll reset into their bores without any other work and amazingly,compared to many other brakes, without causing the other piston to follow through. This alone is worth so much time at endurance events where folk might have to fit new pads before the end of the race. Why anyone would want to use a brake that is harder to fit pads on than the work above baffles me.