To follow up what was already mentioned here:
While only 1 course can be assigned to a section, the course that is assigned to a section is just the first one that students will see when they log in. Students can continue to access any other CSD or CSP course at any time by first typing in the URL, such as:
studio.code.org/s/csd1 for CSD Unit 1
studio.code.org/s/csp2 for CSD Unit 2
Once students visit these units (regardless of whether its assigned) and complete just one level within the unit, that unit will “stick” to their homepage (studio.code.org/home) for them to easily access at any time, as long as that unit is not marked as hidden by their teacher. You can read more about hiding/showing of lessons (separate from assigning of lessons) at https://support.code.org/hc/en-us/articles/115001479372-Hiding-units-and-lessons-in-Code-org-courses. When you change the assigned unit, such as assigning Unit 3, students can still access Unit 1 if they follow these described steps above.
You as the teacher can change the course you’re viewing progress in in your ‘view progress’ tab by changing the dropdown list on the left side to any other course, to see how your students are doing in courses other than the one assigned. Changing the course assigned will just change what students see first when they log into your section. Hope this helps!