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You can change just pads without one, but it's a bit more trouble. Before you remove each pad (separately..one at a time), stick a good sized paint scraper or spatula blade between the back of the pad and the piston. Then push the piston back into its bore so there is enough room for the new pad. [Note: If your pads are very worn, you need to watch the level of the brake fluid in the master cylinder. As you press the piston into the bore, the brake fluid will 'back-fill' the M/C. You will have to remove some of the fluid to keep it from overflowing...unless you've never added any since the last brake job!]
Once the piston is pushed back, hold it ther while you remove the old pad and insert the new one. P.S. If you put any of the anti-squeal stuff on the back and edges of the new pads, do them all first and let them firm up before you begin the exchanging of pads.
That's why you only remove one pad at a time...so that doesn't happen. If you remove the calipers, you will have to restrain all the 'free' pistons so they won't blow out of their bores.