Upgrading diamond question

My fiancé purchased my engagement ring a year and a half before he proposed. Since he purchased we have both moved up at work and are making a bit more money then we originally were. He wants to upgrade the diamond but doesn’t want to go back to Kay jewelers to do that (understandably). Has anyone had good luck upgrading/trading in their diamonds? Where did you go? Did they give you a decent amount off for trading in your old smaller diamond? Thanks!