Hmm, how long ago did you guys try this, because I'm doing it right now and Windows is doing it on the fly at the same speed as my HDR TV. I tested it both in Modern Warfare (HDR) and Pinball FX (not HDR) and Windows switched between both fine. I also tested the Xbox overlay in Modern Warfare and it switched between HDR and non HDR without any bugs or issues. Also non-HDR apps looks fine when Windows HDR switch is "on" but the HDR signal is not live/activated.
Samsung G7 monitor (HDR600 only)
GTX 1080
But as I said, I'm no expert so I might've missed something..
Edit: How the hell do I get Netflix to play HDR on PC.. Tried both MS Edge and the Netflix app, but nope.. (Yes i have Premium / Ultra. Connected via DP of course..)