A gen or gen size bezel will NOT fit the WM9 retaining ring. You will need to start there for the swap.
If the bezel assembly you're speaking of comes with a retaining ring, you're all set.
Speaking from experience (actually by mistake) a utility knive razor blade will not only fit in between the case and the retaining ring, it will pop the ring out of the case. Of course, the way BK presses the cystals in, it's not going to come out of the ring easily. At least not without the proper tools or the knowledge on "how to"...both of which I lack.