This is a very interesting situation and so I’ll try and give you my best advice.
Firstly, I can see why your wife is so fond of her ring(s). Clearly there is an emotional attachment but they are also are very pretty and tasteful, and they look great together. So, the initial thing that you need to ask yourself is whether you need to change the diamond at all.
The most perfect engagement ring in the world makes the hand of the wearer look beautiful. The two complement each other. Equally, on another person’s hand the ring wouldn’t look at good. So you have to think of the ring(s) as only part of the story.
One of the problems of changing the diamond is that you will change the look and the balance of the ring(s). This may be very positive but it can also be negative. I would say that if you decide to do it then you might want to consider getting good advice from a jewellery designer as well as a jeweller. In addition, in changing the centre stone you may find that you need to change the side stones so that it all matches in terms of brilliance (excellent or ideal cut) and colour. You may find that in changing the engagement ring you need to remodel the wedding ring so that the two rings still fit together.
Secondly, you need to consider why you want to change the diamond. Do you want to show your wife how much you value her by getting her the larger diamond that you can now afford? Do you want to impress your friends? Are you avoiding embarassment if your friends are buying their FIs huge stones? Are you simply aware that your wife loves sparkly things and so you want to give her something that she will enjoy?
I would say to you that the best reasons to update the diamond is to show your wife how much you love her and to give your wife the added enjoyment of extra sparkle. Don’t be seduced by the jewellery industry or sites such as Weddingbee. Size isn’t everything. Cost isn’t everything. It’s love that is everything. In a sense you have already given her the biggest, best, most expensive diamond in the world; you gave her all you could afford at the time.
Thirdly, you need to consider whether your wife wants her diamond ‘upgraded’. Sometimes women don’t want this at all. You may have asked her this already. If you haven’t then I would recommend that you do so. You then need to ask her what quality and size diamond she would like. If what she would really like is a 1 carat stone you both may want to wait few years so that you can afford it.
Fourthly, supposing that you do wish to upgrade the diamond what should you do? You have to go and talk to jewellery designers and jewellers to see if it is even possible. Ideally it should be done as simply as possible. Maybe all you have to do is to take one diamond out and put another in. If this is the case then the easiest thing is to get a jeweller to do the work and to source the diamond. This isn’t the cheapest option but it ensures that the responsibility for everything is with the jeweller. They will also be able to match the main stone to the side stones.
If you want to do this more cheaply or get a bigger diamond for your money you need to go to the reputable online dealers such as Whiteflash, Brian Gavin, Good Old Gold, Mervis, James Allan, and Diamonds by Lauren. They have fewer overheads and a greater range of stones to choose from. They give great advice on the 4Cs (cut, carat, colour and clarity). Out of all of these companies I would tend to go to Brian Gavin because he not only sells diamonds but is a diamond cutter too. He won’t sell you a bad diamond because his reputation would be at stake. Out of the diamonds on this site I think his are probably the best. Diamonds by Lauren does coloured diamonds so if your wife has a longing for a yellow diamond you might like to consider them.
If I were you I would ring these companies up and talk to them about what you would like to do and get their advice. They may just want to sell you a diamond or they might be able to swap the diamonds for you or they might give you the names of reputable jewellers or jewellery designers.
You may find that swapping the diamonds is complicated and needs redesigning of one or both rings. In this case, as I’ve said before, you will need a very good jeweller or jeweller designer to do the alterations.
In buying a new diamond you will need to consider the 4Cs and also the shape. I notice that the diamond is square. If you have to maintain the square shape have a look at a few different options such as princess, radiant, asher, emerald and maybe even cushion. Radiants are particularly spectacular, especially with coloured diamonds. You may wish to also get certification. Choose a stone with GIA or AGS certification. They are the most stringent and will verify carat weight, clarity and colour. If you wish to change the shape then there are other possibilities such as oval, marquise, heart, and round brilliants. Again consider certification. An added advantage of round brilliants is that certification will also verify the quality of the cut. This is because the majority of gemologist research has been done on round brilliants and so they have more information on what cut angles produce the best stones.
At this point I have to say that I strongly recommend that you keep the old (current) diamond and maybe put it in a bezel pendent setting so that your wife still has the diamond she has grown attached to. The necklace can also be passed down to other members of the family (children if you ever think of having any).
Fifthly, let us suppose that you cannot change the diamond. What do you do? Well, you either buy your wife a completely different type of jewellery (diamond necklace or diamond earrings) or you buy her a new ring or rings.
She can then has three choices. She can wear her new ring(s) and put the old rings away, perhaps just wearing the old rings occasionally. She can wear her new ring(s) on her right hand while wearing her old rings on her left hand. She can wear her new ring(s) on her left hand while wearing her old rings on her right hand.
If you decide on this approach I recommend that you involve your wife in the choosing of the ring so that you make sure that you choose something that you both like. In general, in considering the diamond I would recommend you choose the best cut diamond that you can. Cut = sparkle and 90% of the beauty of a diamond. Then consider the carat, then the clarity and lastly the colour. Most people cannot tell the difference between a D and an I colour diamond once that diamond is set in a ring. Even gemologists cannot tell the difference between an internally flawless diamond and a VS2 diamond without the aid of x10 magnification. You will save yourself a lot of money by not buying into the myths perpetuated by jewellers. [A lot of jewellers sell D IF stones that are poorly cut. They get them cheap because they don’t sparkle. They sell them on at astronomical prices to unsuspecting customers who think that they are getting perfection when the truth is that they are getting an ugly stone with high colour and clarity that the human eye cannot detect.]
If you really want to choose the best stone then not only choose a great cut but make sure that you have several stones to choose from. Diamonds are like kaleidoscopes. Even with the best cut stones a slight change in angle produces a different pattern of sparkles. Different people prefer different patterns. Your wife will look at all the stones and suddenly say “That’s the one!”.
Finally, take something from the old ring to put in the new. Why not make sure that the surprise diamonds in the old ring are transferred to the new ring? Just a thought.
That’s it. My best advice.
Good luck with your decisions.