When you exchange one to one (i.e., the same number of points) they only need to cancel your reservation not the certificate associated with the award it and can apply the certificate to a new reservation (analogy - when you book a night with a free night certificate and you have to change - you don't get a new certificate just a new reservation). I'm not sure if their system is smart enough to catch "double booking" for the same certificate in real time - this is one way a proactive agent to solve the problem.