In the paper, we study the wavelength assignment problem in waveband switching (WBS) networks, which is composed of multi-granular optical cross-connect (MG-OXCs) interconnected by fiber links. WBS networks with MG-OXCs can route traffic at multiple granularities at the same time; therefore, the complexity of switching fabrics can be efficiently reduced. Moreover, in order to relax the wavelength continuity constraint on lightpath establishments, each MG-OXC node is equipped with certain number of wavelength converters. Focusing on minimizing the extra port consumption and utilizing wavelength converters in an efficient manner, we propose a heuristic algorithm, named Least-Configuration with Bounded Conversion (LCBC), under the dynamic traffic environment. Simulation results show that our algorithm can achieve a considerable port saving and significant performance gain by a well-design cost function.