Thanks Glenn. When I bought XP11, I received the key in a message...
Thank you for purchasing X-Plane 11 as a Digital Download! Your X-Plane Product Key—good for both X-Plane 10 and X-Plane 11—is: <key-sequence>
Since they told me it would work, I figured I should try it. Turns out it was a limited time combo deal from Laminar.
Why XP10? Because I'm getting lots of crashes in XP11, and poor performance on souped-up hardware. I need to investigate whether it's XP11 or my PC. Turns out XP10 works much better on my PC. I found my old copy of the DVDs, so bypassed the download registration for now; but have to keep a disk in the DVD drive as a penalty. XP10 is much more stable, performance much better, and joystick control smoother.