While only 1 course / unit can be assigned to a section and all students within, the course that is assigned to a section is just the first one that students will see when they log in.
If you want students to be able to see and access any other course, just have them type in the URL one time (or assign it to them one time), and have students finish just one level in that unit. Once students finish at least one puzzle in any course / unit, that course will permanently "stick" to their homepage (studio.code.org/home) for easy access going forward.
For example, if you have CSP Unit 2 assigned, but want some students to access CSP Unit 3, they can do so at any time by just typing in the URL, studio.code.org/s/csd3-2020. Or they can find it through the course catalog, studio.code.org/courses (there's a link to the "Course Catalog" atop the homepage as well). They only need to do this once, as after they finish one level in Unit 3 or any other course, it will permanently appear on their homepage for easy access going forward, regardless of whether or not it is assigned.
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 - it is not a definitive constraint as to what courses students do or don't have access to.