Trying to get a custom engagement ring, help?

I have a promise ring from Helzberg Diamonds, it's 1/7 of a carat. We have been thinking about possibly buying bigger diamonds to replace the main stone to make it my engagement ring. Only reason I like the idea is because we arent really having the traditional proposal, etc and it would be very special to have the same ring altered, because the ring itself is very special to me. I love it, I keep comparing other rings to it, but I would never want to just get an engagement ring that looks like it. In other words I only want a ring like my promise ring if it is my actual promise ring. I'm just not sure how much customization is possible? Anyone ever done this lately? My local "major" jewelers are Zales, Helzberg, Marks and Morgan, Kay Jewelers, Jared. Would local jewelers be more willing to do such a thing? Any advice is greatly appreciated!