You can expose the yellowed decals to sunlight to bleach out the yellow. I tape my yellowed decals to the glass of an old picture frame. I place the picture frame in a window with a southern exposure for two to three days. That's usually enough to do the trick. If you have any red in the yellowed decals, you may experience some fading if exposed to sunlight so use your judgement on that.
I prefer to buy new decals if the kit is 10 or more years old. The cost is worth the lack of frustration. Old decals seem to have lots of issues beyond yellowing. They fail to stick or they fall apart or they silver because of the age of the glue on the decal in spite of a coat of Future.