If you own Lumia 520, 525, 620, 630, 635 (512 MB), 720, 820, 920, 925, 1020, 1320 or any other non eligible windows phone that did not get windows 10 mobile, do not make use of WDRT aka Windows Device Recovery Software to roll back. Microsoft no longer offers windows 10 mobile threshold releases for unsupported devices in the release preview ring.
So if you make use of windows device recovery software, you will no longer be able to upgrade to windows 10 mobile again. It will be interesting to see if Microsoft offers something like legacy ring soon for the old windows phone 8.1 users but right now, there is nothing.
To get TH2 or redstone, one had to be on windows 10 mobile threshold build and now that it is no longer available for WP8.1 devices, there is no way to spoof your device either.
So if you have installed windows 10 mobile on your WP 8.1 device, do not ever use WDRT.