Wow, 4wd, no idea what you're doing there. But it works!
And in such short time after my asking.
I placed your batch file into a special reserved location. Created a shortcut to that batch file, and put it into my SendTo folder. Then, selecting a file, right-click to bring up context menu, SendTo your batch file.... Presto. Spaces changed to underscore, right where it sits.
Terrific job, 4wd. Thanks a million!
P.S. -- Why this request to begin with? A finicky program that won't accept being 'sent' a file if there are spaces in the filename, or even in the path. Go figure. But good now.