Nonlinear dynamics in semiconductor lasers subject to external optical injection is investigated for all-optical frequency conversion. No probe or pump beams are necessary using the optically injected system, which would greatly simplify the structure of the conversion system. The frequency conversion can be dynamically tuned by controlling the level and frequency of the external injection. Tens of GHz, or even hundreds of GHz, of frequency conversion can be achieved. The conversion efficiency is observed to decrease with injection level but increase with detuning frequency. On the other hand, the transmission efficiency is shown to increase with injection level but decrease with detuning frequency. Therefore, for high conversion efficiency, no high injection power is required, no serious frequency filtering is needed, and signal amplification is achieved.