OK, thank you for the input. If this is/was caused by compatibility flags being incorrect, what files/changes might GSX have made that I can view to help me understand what I may need to correct elsewhere (in another application, for instance)?
You said you ran the FSDT Live Updater, and it changed the resolution. As I've said, the FSDT Live Update DOES NOT make ANY changes to the FSX.CFG.
It's *possible* that changing a compatibility setting in the registry, might have instead forced the sim to look for the FSX.CFG in another folder and, if this wasn't present before, it created a new one, with all default settings, so you might not look at the file the sim is actually using.
Or, it might have been the opposite: your old configuration was in the "wrong" (because of the wrong compatibility settings) folder, now it's finally in the right one, because the updater fixed the registry, but it doesn't contain your old settings.
That's the only possible reason I could think of, according to your report, of having the resolution changed "just" by running the updater.
If, instead, you ran the GSX INSTALLER, that's an entirely different thing (but it's not what you said), because the installer DOES change the FSX.CFG.
Nothing to do with the resolution, of course. It only set the paths for its own Simobjects, and it ensure you have the "Advanced Animations" flag enabled, otherwise no jetways and humans will work.
So, normally, even if the GSX installer changes the FSX.CFG, you wouldn't see any difference in graphic quality, since it doesn't touch any of those settings. HOWEVER, if your existing FSX.CFG was already corrupted BEFORE running the installer, usually because of having edited it by hand, it's possible that running the GSX installer might have caused further issues, up to a point that FSX decided to reset it to default, because it became unreadable, so it started from scratch with all default settings.
But that would only happen (again, theoretically, and ONLY if your FSX.CFG was already corrupted) if you ran the GSX Installer, not the Live Updater, which doesn't touch the FSX.CFG.