So I select the option to open a Copy, Then I make my edits and Save As using a new name.
One time, after editing, I did Save As and used the original (locked out) document name, but that made me uneasy. OR is that what I should do... Save As the original name and allow it replace the "existing" document? Actually, no, that can't be the solution because the next time I opened that same document, I got the Document in Use error anyway.

Before I click Save As, I look in PROPERTIES / SECURITY to make sure that the READ-ONLY box is not checked. But that makes no difference.
(I see this subject discussed in 2015, but I don't understand it.)
I'm using 4.1.4 in Windows 10.
Thanks.