Double click your master page, set the margin you want there and drag your master page's text frame to your desired width.
The Master does not have a blue Text box, it only has a cyan Margins box (apologies, I don't know the correct terms); so, there's nothing in the Master to adjust. If, however, the Master has the two types of boxes eclipsed, I shouldn't have the current problem, as that is the recommended fix.
Any other possibilities?
Thanks, macinbytes, for taking the time to reply.
If the document isn't sufficiently complex already make a new 1 page document with the correct margins and "Master Text Frame" checked. Holding command or control and shift click on the margin to release your text frame. Dump all your text into the frame. It will automagically add pages as needed and adjustments like this will be a snap. For working with InDesign the way it sounds you are this will pay forward.
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You do not need master text frames, nor are they likely to be helpful. What you do want is Layout Adjustment (Layout > Layout Adjustment...). If turned on BEFORE making your changes (and the margin changes should be done on the master page to affect all pages), any frames that are snapped to the margin guides will adjust tyo the new guide dimensions.
OK, this worked. It really worked! (It's easy to get a novice excited.)
FYI, after I imported the MS Word doc into the new one-page INDD doc you recommend, I  changed the margins in the Master (Layout > Margins...), then  manually adjusted the Text Frame in the master. All text within the main document fell into place. As I need to trial-and-error to get the best text/layout fit for the book, I like the ease with which I can now manipulate the entire 200 pages. (I think there's a "adjust object size" trigger I needed to set somewhere, as a number of object did not adjust.)
Now, if InDesign only handled Endnotes--another query I plan to post--I could skip MS Word all together. How in the world could ID v5 still not handle Endnotes, especially given it handles Footnotes? (rhetorical question)
Peter, your recommendation worked, too (i.e., in addition to 'macinbytes').
FYI, I re-built my INDD file from scratch, and made sure your recommendation was followed: I selected Layout Adjustment before I adjusted margins.
I appreciate your response. Cheers.