No, lesson plans can be adjusted as you go along, but once any assignment is dragged to the Assignment Calendar, then it becomes more permanent and is included in the grading for that course. I would recommend only assigning a portion of your lesson plan at a time in order to maintain flexibility.
Also, you don't have to use all of the items in your lesson plan. If you decide to skip over something, just don't drag that assignment over to your student's calendar. It will remain in the Lesson Plan for other students if needed, but will not be counted in the student's grade unless it is actually assigned.