Samsung unveiled their One UI at their 2018 developer conference alongside other exciting news like the development of the first foldable Android smartphone. At the conference, Samsung confirmed that their latest handsets and tablet (S9, S9+, the Note 9) will sport the new interface. However, today, Samsung confirmed that the S8, S8+, and Note 8 will also receive the One UI update.
We can only assume that the new interface will ship along with the Android 9 Pie update. However, there is no timeline for when either of the updates will ship. For now, we can only hope that Samsung keeps their promise and pushes an update sooner rather than later.