Rerouting is a viable and cost-effective approach to decrease the blocking probability in legacy circuit-switched networks. We study lightpath rerouting in optical WDM networks in this paper. We investigate two different lightpath rerouting strategies, namely, passive rerouting and intentional rerouting. Passive rerouting means rerouting established lightpaths to accommodate new lightpath requests which will otherwise be blocked. Intentional rerouting is to intentionally reroute existing lightpaths during their life period without affecting other lightpaths, so as to achieve a better load balancing. Through extensive simulation studies, we draw the following conclusions: 1) when there is wavelength conversion, passive rerouting works much better than intentional rerouting; 2) when there is no wavelength conversion, a naive-wavelength- retuning algorithm can achieve the most benefit of passive rerouting while path-adjusting does not help too much.