A big THANK YOU to all who provided advice here.
I had completely uninstalled my antediluvian K4PC 1.33, so I installed 2.4.0, ran the batch file to block updates, signed in to Kindle and downloaded a book I bought several years ago.
I used the "From KFX" plugin to convert it to epub, then deleted the KFX format from Calibre to be sure that the book I opened to test was epub. It all worked seamlessly. To borrow from the book in question I need worry "nevermore"
(or at least for now)