Swapping classes is “trading” one class for another without giving up your seat in the original class until your enrollment in the new class is confirmed. When you drop a class and add another separately, you risk not having a seat in either class.
You can swap any two classes (if you meet the prerequisites of the new class) online during the registration periods published in the Semester Bulletin. How? See the tutorial Swap Classes.
Classes that have labs, must be dropped and then added. You cannot swap classes that have labs. This means, you do risk loosing your spot in a previous class.