"The Tolkien estate will insist that the main shape of the Second Age is not altered," Tolkien scholar and series supervisor Tom Shippey told the German Tolkien Society. During this period, Sauron tries to reform Middle-earth for the better but falls into evil, becoming a powerful and oppressive ruler. "Sauron invades Eriador, is forced back by a Númenorean expedition, returns to Númenor. There he corrupts the Númenoreans and seduces them to break the ban of the Valar. All this, the course of history, must remain the same."
Shippey points out that although the broad strokes of the history of the Second Age are established, there are plenty of unanswered questions about the events of the period, such as what Sauron did after the fall of Morgoth. Amazon can take creative license to create its own story within this history, as long as it doesn't contradict Tolkien's writings.
The Tolkien estate maintains power of veto over any content in the show, Shippey says, and is willing to nix anything that doesn't fit with Tolkien's vision. The First Age and the Third Age of Middle-earth (in which the books are set) are both "off-limits" to the TV show, so don't expect to see hobbits, Gondorians or many familiar faces in the new adaptation.
Shippey also reports that Amazon hopes to premiere the series in 2021, but it is still early in the production cycle so this date could be pushed back.