Well, I think sometimes if people marry young they might sometimes settle for a ring that they aren't truly ecstatic about inside--whether they wanted a bigger stone in their hearts, or they weren't actually able to get their "dream ring"......in a lot of cases, they weren't able to financially afford the ring that they truly wanted. Then later on in life, years down the road when careers have changed, maybe they decide they want to upgrade their ring---I think that's the case for a lot of ordinary people not on TPF. I know my mother and father married as teenagers when they were still technically in highschool (no college education) so no huge money was being made there. My father got my mother a pretty, very tiny diamond engagement ring that only costed 100 dollars, because that was all they could afford. Fast forward to a few years ago, she upgraded to a more modern ring with a slightly larger stone.
I know the price of a lot of gorgeous engagement rings shown on here (15k, 25k) are wonderful, but not realistic for my financial goals in everyday life.
It's all in opinion. For me, personally, if I want a larger stone after that day comes then I would upgrade to that. If I'm fine with my ring, then I would just keep it. I think I would only change the center stone though as if I just changed the whole ring it would feel kind of weird, to me.