We propose a novel wavelength-division multiplexed passive optical network (WDM-PON) to simultaneously transmit unicast data and multicast services with upstream data re-modulation in optical network units (ONUs). For each wavelength channel in the optical line terminal (OLT), the downstream unicast data are applied to one arm of a dual-parallel Mach-Zehnder modulator (DPMZM) to generate baseband unicast non-return-to-zero (NRZ) signal. A radio frequency (RF) control signal is applied to the other arm to present two un-modulated sidebands for multicast data modulation in a differential phase-shift keying (DPSK) format. The flexible and dynamic multicast services are realized by simply switching the RF control signal on or off. The proposed scheme is experimentally demonstrated with 1.25-Gb/s downstream unicast, multicast, and upstream data.