So from searching around, I know that there's no need to reactivate when reinstalling the aircraft or sim on the same Windows install, and I know that for a complete reinstall then I do need to return the license beforehand.
What I'm not entirely clear on is how an in place upgrade from Windows 8.1 up to Windows 10 will affect the license, and therefore if I should be deactivating before doing so.
I haven't actually done anything yet, as I'm doing a full disk backup before I touch anything because you can never have too many backups!