EDIT:
I should have visited my son earlier. I saw him yesterday. He figured it out in about two minutes. That's a long time for him -- he's in charge of enterprise software at a LA area college.


...and... it turned out... that I was using the wrong cable type. As soon as my son said "that should work" and it didn't, I inserted the null modem and voila: it worked!

ORIGINAL POST:
I swear I remember there was some way to change the COM port from 1 to something else. Can it be done?

Still trying to make a modern Windows 10 or 7 computer (or even an XP!) able to edit the programming in a Marantz RC5200.