I see this thread "SSH doesn't allow me to modify file"... https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=62443

...and now I wonder how I ever did this in the first place :)

I can certainly go back to making a few changes to config.txt on the Pi itself, but I thought I had figured out a way such as this to store a couple of versions of that config file (eg. one for a CRT and one for HDTV), and just swap them in and out like this without the typing.