This conversation has been merged. Please reference the main conversation: Photoshop CC 2015.1: Preview only toggles once during action
If I open an image, then choose Image -> Adjustments -> HDR Toning... the image appears with the new HDR look.
If I want to see what the initial version looked like, I can untoggle the Preview button on the right of the dialog (beneath OK and Cancel). However, if I re-toggle the Preview button nothing happens until I change another parameter and then the image updates.
In short, the Preview button only works once (when you turn it off) and it does not turn on again when it's reselected. This is intensely annoying.
I'm using Photoshop CC 2015.1.2, 20160113.r.355.x64 on a Windows 7 SP1 64 bit laptop.