Im not sure if a jeweler can take the stones out and fill it in without remaking the whole ring. Im guessing they can, but im not even sure how it would turn out. It probably depends on the jeweler.
I have somewhat of the same problem. I have an anniversary band (half-eternity) it has 17 diamonds and its a size 6.50, im a 5. I took it to 5 different jewelers to ask them about having it sized down. Basically 4/5 jewelers said that it wouldnt be a good idea to have my half eternity sized because it will cause the diamonds to become loose and pop out. The other jeweler said he would size, but it would be flat on the bottom. He also gave me the option of having sizing beads put it or a permanent sizing bar. I decided to try the beads and im going back tomorrow to have them removed. They killed my finger! And there was space in between the ring and my finger. Basically, it didnt look good or feel good.
Im not sure if you will have the same experience with sizing beads because mine are trying to take up a lot of space, so it depends on the size you need. Yours might be smaller so they might not be painful or leave as much space. You should ask a jeweler about trying on a sample. But if it was me, i wouldnt go with sizing beads because this is your engagement ring, and you want it to fit perfectly and fit comfortably since its a forever ring. Plus you dont want to deal with the hassle of removing them if you change your mind.
He explained that the sizing bar would be made by filling in the space between your finger and the bottom of the ring with platinum. So it would still be raised off your finger, you would just see metal though.
i didnt like this idea too much because i like to stack rings and i thought it would look weird next to another ring.
If you do decide to make it a half-eternity band, go to a couple different jewelers. Show them the ring, and tell them your idea. Make sure you tell them that you might want to size it again in the future and see if they would advise against having a half-eternity sized. It may be different with you depending on how much you will size up or down and how the diamonds are set in the ring. If its not that much, it might not alter the shape enough to cause problems with the diamonds. Just make sure you let them know there will be sizing done in the future.
Is there a return or exchange policy where the ring was purchased? Maybe you and your fiance can just order it in your correct size or just purchase a half-eternity in your size instead. If it was me, I personally would keep it eternity since i wouldnt want to lose the diamonds and i would just reorder it in my size. Remember, this is a special ring so you want it to be perfect.
