The usual caution with ABS is to open the bleeder screws before pushing the pistons in, to avoid pushing old brake fluid up into the ABS modulator. Some advocate pinching off the flex hoses while doing this, I personally don't like the idea because the inner liner of a flex hose is actually fairly rigid. Also be careful not to damage the ABS sensor at the hub, and while you're there it doesn't hurt to clean the notched wheel that runs by the sensor - just brush out any gunk that's accumulating there to prevent future problems. As far as reusing your old pads, I suppose you could, but bear in mind your first few stops may be harrowing experiences until the pads are seated to the new rotors. The old rotors will have worn matching grooves in the pads so until those grooves wear down, a minimal amount of pad touches the new, flat rotor. In my starving student days I resurfaced the old pads on a belt sander, just enough to make them flat. A good particle mask is essential, and probably still not good enough for the risk involved though.
-Chris