No Content, no Scenery. I don't know whether it's critical to have GSX to operate efficiently
I'm not sure, but not updating the content, in previous versions, would cause issues with the menus, and since 4.2 had a menu bug that affected GSX, maybe it's required.
But I must say, I don't really know, since I only update P3D by uninstalling it and reinstalling it with the full installer.
If all your addon use the add-on.xml method (like any proper P3D addon should), this won't affect installed addons, and it's usually the most reliable way to upgrade.