I'm surprised at the results; my experience seems to parallel Boraxo's.
Southwest: 100% - always with just 12-48 hours' notice.
American: 90% - adjustment of itin by a day or two.
US Airways: 0% - I don't fly US much anymore, but have a few miles sitting around; try as I might, I haven't been able to find usable open dates for a 40K Europe award.