Thanks for taking the time to help!
I followed the instructions given at
https://www.mobileread.com/forums/sh...d.php?t=140376 to the best of my ability; they weren't very clear to me but basically I clicked on the large "Calibre Library" icon on top and just changed the directory. Once I was in the source directory, I selected all the ebooks with CTRL+A, right-clicked, and chose some copy function presented in the context-menu...in the resultant dialog-box, I checked "move" instead of simply "copy" and that's that.
Incidentally, Windows 10 shows my merged/consolidated Calibre Library directory to have only 1,670 items in it whereas the Calibre program itself lists 1,987 eBooks here. Problem?