This is not a change in 2016; and while some of the specific language originally appeared in the 2015 intervening code cycle update, the underlying excecption for existing buildings pre-dates even that. However, this question always comes up, so I am including it here for reference.
907.4.2 Manual fire alarm boxes. …
907.4.2.2 Height. The height of the manual fire alarm boxes shall be not less than 42 inches (1067 mm) and not more than 48 inches (1372 1219 mm) measured vertically, from the floor level to the highest point of the activating handle or lever of the box. Manual fire alarm boxes shall also comply with Section 11B-309.4.
Exception: [DSA-AC] In existing buildings there is no requirement to retroactively relocate existing manual fire alarm boxes to a minimum of 42 inches (1067 mm) and a maximum of 48 inches (1219 mm) from the floor level to the activating handle or lever of the box.
This exception is proposed for elimination in the 2018 Intervening Code Update. In light of this, it is recommended to relocate existing fire alarm pull stations anyway to ensure future compliance.