Microsoft has sent out messages to those on the Preview program, to let them know that the new UX is on the way:

The New Xbox One Experience (NXOE) is on the horizon! In preparation, we'll soon begin moving all Preview consoles over to the NXOE. If you do not wish to move, please opt out of Preview from the Registration tile in the Xbox Preview Dashboard app.
The NXOE will include a completely redesigned Dashboard, along with various other features, and it seems that everything is still on track for a general rollout in November. But that's not all that Xbox One owners have to look forward to next month - Microsoft will also enable backward compatibility on its new console, allowing users to play games they originally purchased for the older Xbox 360.

Read more: New Windows 10-based Xbox One user experience will soon roll out to all Preview members