The line between "configuration" and "modding" is pretty thin.

One could say that the difference is whether the developers intended the changes you're making to be possible or not, but what about programs with dedicated modding APIs?

At the time (early to mid 1980s) I think we would say "patching". The Wordstar devs certainly did mean you to do this - the memory locations available were fully documented, and I seem to remember they supplied a small patch utility to incorporate your code into the Wordstar executable.