We cruise alot & have a few stores we go to on a regular basis. But, yes, you have to do your homework. We buy most of our jewelry on St. Maarten. So much cheaper than St. Thomas. Most of the reputable shops keep track of what you have bought & will let you upgrade. Majesty Jewelers & Diamonds International lets you do this. They will give you credit for what you originally paid.
Find what you want first. When you are good with the price, then give them the piece you want to trade in.