Correct me if I'm wrong, but you received a code when you bought a new PC, that reimburses or give some discount for a GTX1660. And you are wondering if there would be a problem installing a GTX 1660, which would be a down grade of your current Graphics card. As long as your motherboard is compatible with a GTX 1660 (eg plugs into the same slot), than it doesn't matter what your other card is; the only difference might be a perceived reduction in the graphics quality. My question is, why do you have to install the graphics card at all? Just buy the GTX1660, send out the proof of purchase and redeem your code. Unless NVIDIA has some utility that confirms you have the Graphics card installed, I can't imagine why you would want to do so.