Lesson plans and courses are reusable for all school years. So just start adding lesson plans to the same list and they, along with all of your current lesson plans, will be available when building the learning plans for next year.
If you feel that you no longer need to use a lesson plan, you can drag lesson plans together to create a category. Those can be used to group lesson plans however you want, including into some sort of 'Archive' category. This same trick can be used for courses and resources as well.