Is it possible to make these changes to my engagement ring

My fiancé and I have been discussing getting engaged for a while and had always agreed that when we got engaged we would choose a ring together and split the cost 50/50 (my suggestion, I know I have slightly expensive taste). He popped the question last week and I am delighted, but very surprised as he proposed with a ring he had bought.
He mentioned afterwards that he wanted me to have something to wear once he proposed and we would buy a new one later this year. However, I am feeling guilty as he doesn’t earn much and although it is simple, he must have already spent several 100s of pounds on it. I don’t feel quite right just buying a new ring in a few months time, especially as money is tight for him and it’s the commitment that matters not the ring.
The ring is a white gold band, with (I think) about a 0.1 carat diamond in an illusion setting (picture attached). My dream ring would be the same band, with around a 0.6 carat pear cut stone (would probably go for moissanite), then a pave wedding band.
Does anyone know if it would be possible for me to keep my existing band and have it set with a pear cut stone? Or failing that a similar carat round stone?
Thanks in advance for any advice :) 3A7C0541-AC56-4C54-A4A7-4244379CCB3C.jpeg

Dana ♥

The Centenary Diamond
if money is no object, then with jewelry just about anything is possible.

but, in more realistic terms, the short answer is... it depends.

essentially rings are made one of two ways - to put it simply.
the first way is where stock parts are assembled into the finished product
the second is where the ring is cast as a single piece, meant to hold that specific stone size and shape.

the second way can be either 100% truly custom, unique and utterly one of a kind, or simply a ring made for that size/shape.

If your ring is the first kind - where it is the ring shank that has the head (prongs, or sometimes called claws in the UK) soldered on, then it would not be difficult for a competent jeweler to remove that head and re-set the ring with a larger head to hold a different size, or shape, of stone.

however, if your ring was cast as a single piece, then it would be much more work (not impossible, but may not be much of a cost savings versus buying a new one) to have it reworked to hold a different stone.

this isn't something readily obvious from your photo. if you can stop by a jeweler - a real jeweler, not just a "jewelry store" in a shopping center - and ask them to check it, they should be able to let you know which type of setting you've got.
Personally, had it been my situation, I would probably sell the ring or have the gold melted down and start from scratch that way.
You'd probably be surprised that it can often cost just as much money to have a simple ring like that modified than if you just started over.