It seems it's an upgrade to the whole platform which adds Chromecast remote functionality, and introduces the PlexPass system.
So it might not actually be needed for Now TV users.
Now, Roku users who've already got the Plex app installed have "essentially been gifted" the Plex channel. As the Now TV box is based on the Roku LT, it's possible that the same applies for our little white box. If you find you are asked to purchase a PlexPass after 30 days, just roll back to 2.6.9