Is it really as easy as removing those two bars with little screws on either side of the glass when you have the flip up, eye-killing cover open? I'm considering replacing mine as it's probably nearing 100 years old and I've heard new GGs are brighter. I work a lot indoors and in lower lights so anything to brighten up the image would be cool.
If you haven't figured it out already, you can remove the flippy cover by just pushing out on the tabs on each end (at least on my graflock version) Then all you need is a small screwdriver, and what Argus said if their is a fresnel. Although Argus is to tasteful to shill it directly, he makes and markets a fine GG.