I think that if this message appears directly below the Revert to Legacy Dashboard button (as shown in your screen shot) then "in the top right hand corner" isn't needed. On the other hand, the screen shot shows that when you're on the Legacy dash, you are provided with a link in the message as well as a button to return (line 137). Maybe provide a link here too? (it's parallel UI and if we really really want people to use the non-Legacy version,  let's make it easy, right?)