After further fare searching I see that part of the problem I experienced looking for low fares today is that SW's 7-day advance fares, at least on several AUS routes, spiked sharply (more than 30%) on Saturday 26-Apr. Two examples:

• AUS-DEN was $87, now $119
• AUS-PHL was $81, now $109

I hadn't been looking at other 7-day advance fares recently, so I don't know whether other markets might have seen similar increases. Friday night I booked PHL-AUS for 02-May—which despite meeting the the 7-day deadline did carry a Friday night premium. I was expecting, however, that I'd probably change it soon to the $81 fare available on 06-May or 07-May. (I'm waiting on schedule coordination information with others assisting with a family health issue.) I guess I can think of is a "change fee bargain, only $28!"

I confess that I did think the old fares were amazingly low for such short notice.