As explained everywere on this site, all our products are sold in a single version, there's no "FS9 licence" vs "FSX license", you just bought the scenery and you can use it in both sims.
So, if you install the FSX version after having registered the FS9 version, on the same PC, the FSX version will be already registered.
If, instead, you want to use the FSX version on a different machine, install it there, then simply use the "Register Serial" feature to activate your Serial Number (found in your email receipt) on the new machine.
Thanks, I didn't see it on your site. That is why I posted in this forum, I looked on your site first.