I've been thinking about this a lot lately as I'm in the process of upgrading. Like others in this thread, I always said I would "NEVER" do it and that it is silly. My original ering was absolutely beautiful - a high quality diamond in a unique and beautiful (to me) setting and a matching wedding band with diamonds set inside a channel. However, the setting always was compromised as the jeweler who sold it to us did a resize on it for us that he probably should have refused to do and it was very poorly done. We always planned to have the setting worked on someday when we could find a good artisan we could trust.
Fast forward fifteen years, many personal/relationship hardships, career changes and growth, a major move looming in the future, recent deaths of beloved parents and all the hard work that went with their care and now settling their estates, etc., etc. We took my late MIL's jewelry into a jeweler we know of who is also a fine artisan to have it appraised and also talked with him about fixing my setting.
It was my husband who broached the subject of upgrading to a larger stone while we're at it. So we asked the jeweler to source some for us to "just look at". And okay, I admit it.... as much as I loved and will always love my original stone and truly had no desire to upgrade it, yes, I can be materialistic and I fell in love with the idea of having a larger stone on my finger. We're keeping the same shape of stone and the same quality and we're changing the side stones (which were always ugly and we never liked them in the setting) to nicer quality stones of a different shape. Otherwise, the setting and wedding band will remain the same, just more structurally sound. I will probably try to sell my original stone (as much as that pains me) because I try to be frugal even in my luxuries and I want to lighten the load of the upgrade even though we can well afford this.
I've had a lot of mixed feelings about the whole thing and I kind of feel like a traitor to my original stone. But my DH keeps reassuring me and reminding me it was his idea and that he knows I will love my new ring and be so happy with it and that this is a fitting time to do this as we're coming up on a milestone anniversary, the loss of our loved ones and a big move to a new state and into a new home that we are building together, and basically it is as if we are reaffirming our commitments to one another and starting a new chapter of our lives, so this is just one more thing we are changing and improving TOGETHER. When he put it to me that way, my heart just sang. I am indeed a lucky woman.
Like others have said in this thread, it is all about the RELATIONSHIP and how meaningful it is to you. The quality of the relationship is not defined by the quality of the ring. The ring is a symbol and you can choose to recreate your symbols or leave them just the same - what is important is the reasons why and what it symbolizes to you.