why overcomplicate shit? push the pistons in with the old pads in and then change, who cares if you f-up old pads?
Have you, or someone who thinks they are helping you, ever closed the brake lever with the wheel off? Are you going to put the old pads in and then take them back out again? There is a need for a tool in some cases.
a hunk of wood rubber-banded between teh grip and the brake lever prior to wheel removal has prevented this for me on many occasions. It's definitely overkill, but especially when I am leaving the wheel off for an extended period of time (fork maintenance, etc.) it has kept me from being an idiot more times than I care to admit.
Buy a pack of Cedar door shims...great for spreading the pads apart without any damage. They also work great for scraping mud off my mx bike between motos. Should be under 3 bucks a pack (10 count).