We propose and simulate a simple scheme of all-optical format conversion from non-return-to-zero differential phase-shift keying (NRZ-DPSK) to return-tozero differential phase-shift keying (RZ-DPSK) by using phase modulators and detuning filters. The operation principle is theoretically analyzed and simulated by exploiting spectra, temporal waveform and eye diagram with commerical optical design software VPI Transmission Maker 8.5. The use of electrical clock recovery and linear phase modulation in the conversion scheme may be potiental in practise use.