Verbatim from UA almost a year ago, emphasis mine:
We also plan to purchase 50 additional Boeing 737-900ER aircraft with deliveries beginning in late 2013. The new 737-900ER will also feature the Boeing Sky Interior, which has been included on our 737-900ERs since 2011. These new 737-900ERs will be used primarily to replace our older, less-efficient domestic Boeing 757-200 aircraft.
https://hub.united.com/en-us/News/Co...737-max-9.aspx