Yes, you can keep your present ECS-ID and the password.

In April, you will receive an ECS-ID notification from your department on which the same ECS-ID and its activation key are printed. Unless you forget the password, you don't have to activate it again. In case that a different ECS-ID is printed on the notification, please visit the ECS-ID help desk at Academic Center for Computing and Media Studies, South Building with your new ID card issued by Kyoto University.