OP specifically asked for a device which was in 8.1 and then got upgraded to W10M.
Some phones come with W10M out of the box. Those are the phones that won't be able to rollback to W8.1 (Because they don't have 8.1 ROM).
By saying that
If the official W10M build for your phone is out then no, you won't be able to roll back so the WDRT won't help.
You are clearly saying that the 8.1 devices which got the official (non-insider) W10M update won't be able to rollback.
The servers still don't have the full W10M ROM complete with the updated firmware for devices that were in W8.1.
Yet the 640 XL has an official Windows 10 Mobile release, just because it may not be available yet in your region doesn't make my statement any less correct.
Just so you know, my 640 XL is on W10M via upgrade advisor.
If your device has an official, non preview, release of W10M then you cannot roll back to WP8.1, the firmware and the OS are not the same thing.
My device was on non inisder / non preview release when I checked with WDRT.
And I am very well aware that firmware and OS are not the same thing.
8.1 is still available on servers on WDRT. Those who want to rollback can do it.
When the complete W10M ROM (OS+firmware) is out on the server, then you wont be able to roll back to 8.1
W10M is only an OS update now without the firmware part. That is why there is no W10M ROM on the server yet.
Whereas the 8.1 OS ROM is complete with firmware.