If you had obtained the preview release of Windows 10 Mobile, then it would have been fairly easy to switch to regular production builds of Windows 10 Mobile after the upgrade was officially released. At that time, it was fairly easy for Lumia 635 owners to upgrade to the preview release of Windows 10 Mobile because Microsoft hadn't yet officially decided yet to drop support for these phones. It would have been much easier if you had enrolled in the Insider preview program several months ago, before the official launch of the public upgrade to Windows 10 Mobile.
