Thanks, UAPremExecflyer. I appreciate your advice. I did search the UA forum but as a relative newbie to flyertalk, I find the search engine difficult to use productively. I'm learning. So, for what it's worth to others who may happen to stumble on this thread:

1) I called. UA rep said that the fare is guaranteed but the seats aren't until SAA codeshare airline confirms. So if there are seats, the fare stands.

2) I got an email 24 hours later confirming my flights and the original (lower) fare.

3) I'm positively convinced that my searching all over the place (Expedia, Travelocity, SAA, United, etc) triggered a rate increase. Only the date that I checked increased the rate; every date for the other 29 days of that month came up with the lower fair. When I purchased the tix, there appeared to be not 1 seat sold on that flight as I chose my seats. There are now 6 seats taken, mine. Take from that what you will.