When you reach the last lesson of a curriculum (Example: Lesson 35 - Module Review), the Explore the next curriculum button will appear.
Clicking that button opens a window with information about the next curriculum: the school year it corresponds to, the curriculum's name, and a brief description of what students will develop. To move forward, just click the Go to [next year] button (the button text changes depending on the year).
You'll still have access to the results from the previous curriculum's lessons and can easily navigate between curriculums. Your students will continue to have access to all the content, and there's no issue if some students haven't finished the previous curriculum yet.