You just have to try the least conspicuous area with an eraser wheel and see what happens. It's the only thing that will work on really degraded stickers.
They don't settle into factory clear, but the paint under the decal can definitely have a different appearance than the rest once polished, I have seen that many times. The pigments age differently because the decal was absorbing UV. I guess if they were applied over some shoddy repaint, all bets are off.