Abstract : In this report, we study the problem of rerouting a set of lightpaths in WDM networks. The reconfiguration issue arises for instance when it is necessary to improve the usage of resources or when a maintenance operation is planned on a particular link of the network. In order to avoid service interruptions, old lightpaths should not be torn down before the new ones are set up. However, this may not be possible since establishing the new routes of lightpaths may require the release of resources previously seized by old routes. Then it could be important for the operator to minimize 1) the total number of temporarily disrupted lightpaths, and/or 2) the number of concurrent disrupted lightpaths. In this paper, we study the tradeoff between both these conflicting objectives. More precisely, we prove that there exist some instances for which minimizing one of these objectives arbitrarily impairs the quality of the solution for the other one. We show that such bad tradeoffs may happen even in the case of basic network topologies. On the other hand, we exhibit classes of instances where good tradeoffs can be achieved. Finally, we investigate instances from various networks through simulations.