I suppose there is theoretically a chance that you could cancel & then have the tickets sell before you can repurchase, but I’ve never had an issue. I just make sure I repurchase right away. And the site will usually tell you if there are only a few remaining tickets in a particular category (wanna getaway, anytime, or business select). When I did mine today it told me that our flight down only had 4 of the business select fares left at that price, but I had no problem cancelling our existing reservation & then booking 3 of the 4 business select fares that were available at the promo price.