Sandra, I had a similar project once and what I did was create a lip
along one side of the internal guard ring to prevent the ring from
slipping off it’s guard. If worn with the lip on the top of the ring
the ring was trapped between the base of the finger and the lip.
Also, it would be a good idea to make the internal ring a very tight
fit to the inside of the original. Another idea would be to create
your inside ring of bezel silver which was wider than the origianl and
literally “Set” the ring in its new seat. You could even do pierced
work to line up with the openings behing the stones.