At least there's some good language if they do add a larger airframe.

All 737 aircraft that do not require a new type rating are allowed, subject to the following restrictions:

ANY aircraft (including the MAX) that is acquired or configured in excess of 175 seats constitutes a reopener or
Any aircraft that requires a new type rating requires a reopener
Clarified “Company shall not place such aircraft in revenue service until such rates are negotiated.”