A big thank you to Leon (his user name) who took the time to PM me and lay down a detailed explanation of how to change the sight blade. With this lengthy description and his encouragement, it took about ten minutes to do the task. What a generous community this has been!
I hope he doesn’t mind my sharing his step-by-step instructions in case anyone else searches out this information. I’ll add a couple of my own notes at the end:
“Step 1 - turn the rear sight elevation screw counter-clockwise until it comes out. If you are worried about parts flying off, don't , they normally don't - but cover everything in a zip lock bag if you are.
The elevation part of the rear sight is now loose
Step 2 - Using a small punch, knock out pin (1203170) from the underside of the sight. It is a tapered pin hence the direction - from bottom to top. You will need to put the rear sight on a small block of wood to do so. I have changed a few of these so don't normally remove the entire rear sight before doing so.
As a first-timer, it may be easier for you if you do. Just unscrew the big screw (1200020) and the entire assembly slides off.
Step 3 - Now turn the windage screw counter-clockwise until it comes out. You can now remove the rear-sight blade and replace it with a new one. Again, nothing is going to fly-out but, if you are worried, keep it covered in a zip-lock bag.
Re-assembly - essentially in reverse order.
Step 4 - Insert new rear-sight blade then screw the windage screw back in.
Step 5 - Whilst keeping some tension on the windage screw ( keeping the rear-sight blade in position ), re-insert pin 1203170 from the top. The reason why you need to keep some tension on the windage screw and the blade in position is that this pin needs to engage with a groove in the windage screw. This stops the rear-sight blade coming loose in case you adjust the screw too much.
Step 5 - Elevation screw - re-insert and screw into place. Special Note - Both the elevation and windage screws have little plungers which makes a clicking noise when you turn the screws. You may have to gently prod them in place with a small punch when re-assembling.
Step 6 - Put the sight back on the gun and tighten screw 1200020.
It may be may be slightly daunting the first time but it's not hard. Should take you about 3-5 mins from start to finish.“
My notes: in removing the elevation screw, indeed there is nothing that will come flying off, but there is a spring and a female receiver for the screw that will be able to slide rearward and off once the screw is removed. You should either tape those securely down in place to do your work, or take note of how they are installed and remove them to set aside to reinstall when you are done. The only other stalls I came up against was in screwing in the new blade take note that there is a slant to the top surfaces meaning there are two ways to install it. On mine the slanted surfaces align with the sight body top when the number on the blade (4.0 in the case of mine) faces forward. Also a bit tricky is realigning and inserting the unthreaded pin section at the end of the screw into the axis receiver hole in the opposite (from screw head) side of the sight body. This was just a matter of patience, a few gentle taps with a plastic hammer. Otherwise it all seems to have come together well and the two adjustments seem to function as they should. Thanks again to Leon!