If you need to reinstall FSX, you simply reinstall the scenery, that's it. Reinstall FSX doesn't have any effect on your activation. The activation is stored in your system, not in FSX and it's entirely unrelated to whether FSX is installed, reinstalled or not.
If you need to reformat or make major hardware changes or install an entirely new PC, you simply reinstall the scenery, then reactivate it using the "Register Serial" function of the Addon Manager. AND, if you use the "Deactivate" function of the Addon Manager BEFORE reformatting or upgrading/changing your system, you won't even consume an activation. If you don't do that, you will consume an activation, but you have 6 of them, before having to contact support for a manual reactivation.
