Abstract Flexible migration that reuse the deployed infrastructure and colorless ONUs which avoid inventory problem are two major challenges of next-generation passive optical network (NGPON). In this paper, flexible migration is achieved by the coexistence of legacy PON and NGPON and colorless ONUs is achieved by upstream remodulation. Orthogonal modulation methods such as DPSK, PolSK and WSK are applied for NGPON downstream signal to ensure the constant amplitude. The amplitude-constant NGPON downstream signal induces low crosstalk to the coexisting legacy PON signal and the remodulated upstream signal. The experimental demonstration shows error free transmission and verifies the feasibility of the proposed scheme.