I'll probably have to pick up a set from Indycals as well. The kit decals are pretty yellow now. I've seen people try to remove the yellow by taping the sheet to the inside of a window and trying to "bleach" out the yellow. I've never had success trying that though.
I've seen it done a couple of times, and it does work, but as Joel said, the risk is ending up with brittle decals.
A funny story from a friend of mine, and a trap for young players. After a week taped to the window in mild conditions, the attempt to remove the sheet from the window ended up leaving a very neat set of decals stuck to the window due to condensation on the inside of the glass !
If you are going to try this, do it with the decal sheet sealed inside a ziplock bag