There are a few issues using old pads:
1. If your using your old pads which have shaped around your old worn disk, especially if its very out of straight will then, firstly take a lot longer to bed in, (not a biggy)
2. My main reason that i dont like using old pads is that the pad will wear the disk unevenly if its not straight, now a rear disk on a supra normally fouls on the inside edge, (the disk face that you cant see) so that normally ruins your pad, which in turn, ruins your new disk, now why would you want to do that to a new disk?
If you sand down your pads surface and make them 100% flat, then in theory they are pritty much as good as new. So if you have a good amount of life on them (over 3/4) then its worth doing, if not, just bin them
Or if you are using grooved disks, they will do the sanding for you too but i would still sand them staight anyway.
Good luck, I have some worn pads if you want them, LOL