Once, I replaced a "stuck" caliper and didn't replace the brake lines. The problem returned - probably the caliper was just fine and the issue was an internally collapsed brake line preventing the pads from retracting correctly. So make sure to replace the line as well.
On my Odyssey, I replaced a bad rear caliper (and brake lines!) and didn't do anything with the pads and rotors. I had caught the problem very quickly, though, so the "stuck" caliper hadn't caused excessive wear. And then I replaced the caliper again when the original rebuild leaked all over. Still kept the pads and rotors.
In your case -- I would assume the rear brakes didn't wear much in 20,000 miles. If this is correct - I suppose it depends on your driving habits - I would not replace pads and rotors on the opposite side. Personally, I would want to have the same brand pad on both sides, though.