Can a company legally impose an expiration date on coupons initially created without an expiration date?

Southwest Airlines recently announced they are changing the way they issue drink coupons. The current coupons have no expiration date. Beginning August 1st, all new coupons issued will contain an expiration date. That’s all well and good. What is not well in good, in my mind, is the fact that they also announced they will stop accepting the current, no-expiration-date coupons on August 31, 2011.

My question is, can they legally do this? The way I see it, the coupons were produced without an expiration date, and that basically forms a contract between them and the customer that they will be accepted at any time. I am looking for any sort of legal decisions or laws that might support (or refute, I suppose) my point of view.

And before you make this comment, yes, I understand that I’m talking about a coupon for a $4 alcoholic beverage, and that in the grand scheme of things this is really unimportant. I’m simply curious about the legal aspects of this decision.