If it's an advisory you could probably get away with slightly pushing the piston out about 3mm with the brake pedal (no more than that), giving it a good clean with a toothbrush and brake cleaner spray (nothing else), putting Red Rubber Grease on the exposed piston, and also cleaning up the pad pins so the pads can slide easily.
One of those things that sounds very straight forward, and it is, but only after you've done it a couple of times.
I clean mine twice a year, and regrease them, and they function really well.
A little time consuming at first, especially the front calipers, but well worth it and also satisfying that you've done it yourself.
You only need to change the pads if they are worn down or in poor condition.
I prefer to use SBS Ceramic pads on the rear as they aren't as dusty as some others.