I personally have replaced disks without pads, and didnt notice any loss of brake performance... Just give the old pads a quick lick over with sandpaper to even them out.

I know its not ideal and is often frowned upon by people, but i had a 10% worn set of £100 pads and i wanted to upgrade to drilled discs, so i done it with the old pads and it was fine.. If we was talking about a £20-30 set of pads i'd have just renewed them aswell!

If you really dont want to spend out on new pads then just re-use the old ones...