In an open-loop unidirectionally coupled chaotic system composed of an external cavity distributed feedback (DFB) semiconductor laser and an isolated DFB semiconductor laser, frequency detuning between two lasers can be controlled accurately by adjusting the bias current of transmitting laser carefully, and then the chaos synchronization state under different frequency detuning values can be investigated. The experimental results show that the system behaves with complete chaos synchronization (CCS) for frequency detuning within a small range (-0.19~0.95 GHz), where the highest correlation value of CCS is 0.84 and the highest correlation value of generalized chaos synchronization (GCS) is 0.78; once the frequency detuning is beyond this range, chaos synchronization will be switched from CCS to GCS. Therefore, synchronization switching between complete synchronization and generalized synchronization can be achieved by adjusting the bias current of transmitting laser. The theoretical simulations exhibit the similar variation tendency as that of the experimental results.