You probably need to settle for the exchange, at least initially (see below). This is a breach of contract suit--i.e. that they didn't sell you the ring they had agreed to sell you. When the other party breaches a contract or agreement, they generally are given an opportunity to "cure," or correct/make good, the problem. So they should have the opportunity to fix this--but if the replaced ring still isn't right, then you could sue them, and likely for fraud or theft by deception as well as breach of contract.On a different note: please accept our congratulations on the engagement.