Originally Posted by nsx

From http://www.southwest.com/rapid_rewar...hip_rules.html item 7:

Reserved boarding privileges will not be provided in instances where the Member’s reservation meets any of the following criteria: reservations booked less than 36 hours prior to scheduled departure; changes made to an existing reservation within 36 hours of a scheduled flight; standby travel on a different flight than what was originally confirmed; flights taken after a Member’s status expires from our A-List. Irregular operations may also affect a Member’s benefits on Rapid Rewards’ A-List boarding priority.

Translation: any changes within 36 hours and you are SOL.