Well, with further tinkering, I see that I can choose persistent settings in the Preferences dialogs.
But I could swear, that previous versions would let me make and keep settings from the Conversion dialogs directly. I don't think I ever set those up in Preferences, only in Convert Books. And they stayed put.
Maybe I'm having one of those senor moments.
Thank you to all who responded, and so sorry for the false alarm!