Sorry to bump an older thread, but I found this in the document now, and this thread came up from a google search.
Changes to Award Travel
...
Changes to outbound Award Travel must be made 72 hours prior to the departure time of the currently-ticketed outbound flight. Changes to return Award Travel must be made at least 72 hours prior to the departure time of the currently-ticketed return flight. Award Tickets booked within 72 hours of departure are nonrefundable and cannot be redeposited or changed.
So the 72 hour return rule should allow changes even prior to outbound departure on the return segment.