How do I convert H.264 Rec709 ("SRGB") to H.265 HEVC 10-Bit Rec2020 ("Wide Gamut")?
Download the relevant build for Windows, eg: https://sourceforge.net/projects/ffmpegwindowsbi/files/july-01-2016/

Use the following command line: ffmpeg -i INPUT.mp4 -c:v libx265 -crf 20 -vf eq=gamma=1.15:saturation=0.925:brightness=0.05:contrast=1.15 -color_primaries bt2020 OUTPUT.mp4

Note that you will have to adjust the various values such as brightness, contrast, saturation, etc. to your taste since this is a "brute force" conversion to wide-gamut Rec2020. This has been tested on iPad Pro with H.264 movie trailers. On wide-gamut displays such as the iPad Pro you should notice richer, deeper colours.

Note that this is not an archival or permanent solution as Rec2020 is a very wide gamut vs Display P3 (latest Apple devices) or Adobe RGB (pro wide-gamut monitors). Newer Ultra HD TV monitors or reference monitors that can get close to Rec2020 may be more suitable.

In any case I think this is an interesting experiment and learning experience for any amateur or pro who has just started exploring wide-gamut and/or HDR video... it is a very, very different world to SRGB that had been with digital media for decades. All that is about to change radically in the next 10 years.