It is way harder to get the rear sight off than to put back in. First, the sight should be driven out from the right side of the pistol. The dovetail is slightly tapered. Right to left out is the ticket. Before you allow the sight to slide out completely, I recommend you put it in a plastic bag and then remove the sight. as you have discovered, the firing pin block, firing pin block spring and firing pin block detent are held in place by the rear sight. If the sight slips past the firing pin block detent, both it and the spring will end up where all small gun parts go, hence the plastic bag.
Once the rear sight is off, just start the new sight in from the left side until it is at the firing pin block spring and detent. Carefully push down on the detent with a small flat blade screwdriver until you can push the sight over the detent. Then, it is just a matter of sliding the sight all the way in.
If your sight is anything like mine was, you will have to beat the living begeebers (technical gun term) out of it. A brass punch is required to prevent marring the sight.
It really is easier than it sounds, so go for it. Just remember the plastic bag.
Good shooting,
Mike
