@JanetH, your posts left me confused about which airline you were actually flying. I saw ways to read it that you were flying Southwest, as well as ways to read it that you were flying Delta. Then, David Ock seemed to me to read it that you were flying Southwest, so I went with that.
Anyway, I wonder if the change of airplanes could have been due to the need to inspect the engines on Boeing 737s. Splitting up people who bought their tickets at the same time, and presumably paid for them with the same bank card, is incomprehensible.
Our first choice is always to fly Southwest. Unfortunately this time we had to fly Delta to Key West which prompted the comparison to Southwest. I wish Delta would handle their seating like Southwest does.