We can certainly help with a redesign, we do these sort of project quite often!
First step would be to bring in anything you'd like to use in the new design. We'll choose which pieces have the most sentimental value and make sure to incorporate those stones into the design.
Once we have an idea of the stones we're working with, we'll get a feel for what you like and create some CAD mock-ups for you. When we finalize a design we can do a 3D print of the ring in wax for you to try on.
As far as unused diamonds/stones, I generally recommend you hang on to them for a future project. If you don't want them, we'll assess them and give you a trade value toward the new piece. The left over platinum would be the same, we'd give you melt value for the platinum to use towards the redesign.
Please feel free to pm me or email me at email@example.com
if you have any questions or would like to come down to meet with one of our designers.