Man-of-Leisure It is backwards compatible when planned for. For example, if the optical muxes on either end don’t have the optical channel ports for the NG-PON2 frequencies you have to replace them, taking down all customers in the mux and splitters attached during their replacement. Verizon decided to use open fibers to a new mux and splitter instead which takes more fibers and space.
OLTs are another matter. The ones originally installed for BPON or GPON in the COs weren’t designed with NG-PON2 ports in mind so they need replacement or new units installed along side the older ones. Same with the ONUs (ONTs) at the customer home.