Check the Waitlist Before Switching
When switching out sections in general make sure you check the section you are wanting to see if it has a waitlist and if there are any students on it. Some classes will appear open but are really not. The seat that appears open is actually reserved for students on the waitlist.
If you try to register for a class that appears open but has an active waitlist, you will only get the option to waitlist for the course. So don't drop a class that you are already in before checking the other section entirely out first.