You waitlist a discussion and then if you get into discussion, you get into lecture.
If you really need math 1A this semester, I don't recommend you dropping your spot in another lecture... there are ~25 people on Borcherds' waitlist with every discussion full. The only discussions without anyone on the waitlist are MW 8-9, 2-3, 4-5, or 5-6.
You'll have to drop Math 1A with Crist first, then add Math 1A w/ Borcherds. It should then say "this class is full, would you like to add to waitlist" and then you select which section you want to waitlist.