Have you fired your new revolver yet? The shape of the grip is intended to prevent the trigger guard from rapping your fingers during recoil, and it works well.
If you prefer the shape of the classic wooden grip that allows you to place your hand higher, you might do better to purchase some and save your originals. Sanding the plastic is possible, but the resulting finish is not pretty.
Many people with the original grips spend quite a few bucks for a grip adapter to fill that open space. You could certainly be different, but I suspect you will be disappointed if you sand your original grips and then need to replace them because they don't work as well as you hoped.
Best of luck!