Lol, forget it . Nougat MIUI9 isn't even out yet in a stable form.
If you're after Oreo then go for the Mi A1 or install Lineage OS when Oreo is available for it.
We all know MIUI is great but is slow with bringing out the latest Android versions.
To be fair, the recent Android versions don't have enough different to warrant a major version change anyway so doesn't really matter in my opinion.