I've done all of those things recommended above, including emptying the trash, which I usually do routinely several times a day. Nothing works, it still tells me I have X-Plane already installed. So I've re-installed the original that I have, as a second installation. Haven't tried it yet, but I'm hoping it will work.
Incidentally, my Regional edition (North America) isn't supported by Randy Witt, at least not in this instance. He told me to contact Graphic Simulations, who he said does the Regionals and who he thinks uses a different installer than the Global edition uses.
Thanks, by the way, for all the advice offered above.