Flexible PON are future paradigm in parallel with flexible and elastic optical networks are under research for core networks. In the same way as those backbone optical networks can be significantly improved by following software-defined network (SDN) techniques, it is described how SDN PONs can be implemented by highly spectral efficient digital modulation formats. A main challenge is the implementation by cost effective devices. We will show the progress in alternatives implementations and adequacy of diverse modulation formats to cost effective bandwidth limited optical sources and receivers.