It's actually my understanding that when decals yellow, they are still usable. I assume it's the the clear film, and not the actualy white pigment that has yellowed? I think it's actually the carrier chemical that yellow. Test a decal, try one of the little ones on the bottom of the sheet where Tamiya write the model it belongs to and all. If it sits clear on the model, good to go. If not, back to plan one.