Seriously buy the new repro sleeves, they're great.
As for getting old labels off cases, goo gone or whatever knock stuff + soap and hot water does the trick.
I usually take a cotton ball, dab it in goo gone and wipe down the label, let it sit for about 2-3 min. Peel off. Dab some goo gone on the area again, let it sit a few more min. Then soak it in warm/hot water. Finally scrub the remaining residue off with a cloth. I finish off with drying the case for what seems like a thousand hours bc ocd oops lol.
Hope this helps at least somewhat.