No, I'm pretty sure that once the weather has been updated to FSX you don't need to run REX in the background any more.Keep in mind though that the weather will from then on stay static and will not change.Best way to confirm this is to try it out, i haven't specifically confirmed this myself but I use this technique all the time and I haven't notice FSX reverting back to the old FSX weather assets.I think I might try this tonight when I get home. I'll let you know what I see / don't see!