You don't! But you can avoid losing them by gradually drifting off the original Rear Sight, from left to right, by doing it gradually in a large box to contain the mayhem. Your workbench should also be clean and un-cluttered. You can definitely do it yourself if you are careful!
After you've moved the PT92's Rear Sight about 3/8's of an inch, you'll see the top of the Retention Cup and then you have to be more watchful.