I haven't done it myself, but I read the FAQ on their website, and from what I understand you can return them, and will get get refunded as soon as they have resold the tickets to someone else. Considering it's sold out for a year in advance I don't think that'll be a problem. I think all performances have wait lists, so returned tickets should be snatched up the moment they are returned.
If it really works that way the system is awesome, it's pretty much the reason I felt comfortable buying tickets 18 months in advance without having any clue where I would even live by then, or if I'd be able to go. Only 8 months away now (lol) and it's looking like I don't have to use the return policy, but it's still nice to know if anything comes up... or I enter & win the Friday 40 at some point before.