Short answer: it is always safe to let the X-Plane updater overwrite core files with SkyMaxx Pro installed. Longer answer: When the SkyMAXX Pro plugin is activated when you launch X-Plane, it copies the sky_color_*.png files inside \\Resources\\bitmaps\\skycolors\\ into \\Resources\\plugins\\SilverLining\\skyColors\\Default Sky Colors\\, and will copy any custom sky colors you've specified in their place. If you've left SkyMaxx's sky color setting at its default, it just copies the default sky colors back in, and nothing actually changes in your X-Plane core files other than their creation dates. However, that can trigger a warning when upgrading, and it is safe to let the updater overwrite them. So, \\Resources\\bitmaps\\skycolors\\ is the only directory we mess with in normal circumstances. There is also a rare case (in fact I'm not sure it ever happens at all in practice) where if our attempt to disable X-Plane's built in clouds fail, we will temporarily back up the contents of Resources\\bitmaps\\world\\clouds\\ into \\Resources\\plugins\\SilverLining\\savedClouds\\, replace them with 100% transparent textures, and then restore the original contents when the plugin shuts down. Hope this helps.