I remember editing command.com too, with debug.exe, to change some of the standard messages, or even command names, as you did.
Another cool DOS thing was that you could type Alt+255 (last 8-bit character code) as part of a filename. That appeared like a space character, so made it effectively "hidden" to those who did not know about the trick, because they could not type its name to list or edit it.
I used to do this on Windows XP to make my desktop look cleaner, all the files were ALT+255 repeated N times, or, more accurately, repeated a random number of times.
Another cool DOS thing was that you could type Alt+255 (last 8-bit character code) as part of a filename. That appeared like a space character, so made it effectively "hidden" to those who did not know about the trick, because they could not type its name to list or edit it.