When you call to make the change, take this approach: "I'm thinking about changing my ticket [describe change here]. If I did that, would there be any additional charge? How much?" That's what I always do on any ticket. If you call and say, "Hi, I'd like to change my ticket to....", you could end up with an agent who, in his/her zeal to provide good customer service, makes the change before you understand the cost. Then the space you previously had may be unavailable. Overly cautious, yes; but I always prefer to avoid rather than solve problems.