Rerouting is an effective approach to decrease the blocking probability in legacy circuit-switched networks. In this paper, we consider intentional lightpath rerouting in all-optical WDM mesh networks. We propose a dynamic least congested routing (DLCR) algorithm, which dynamically switches the lightpath between the primary route and alternate route according to the network traffic distribution. Extensive simulation results show that DLCR algorithm can achieve much better blocking performance than traditional routing algorithms, including shortest path routing, fixed-alternate routing, and least congested path routing. We also find that the performance gain is more significant when wavelength conversion is available.