[racket] overriding unit functions with other units

I'm sure the answer is in the docs somewhere, but I can't parse it out.

I have a signature that provides a standard API and then a unit that
provides generic support for the API.

Now I want to provide support for more specific implementations.
Conceptually, what I think I want to do is import the generic unit and
then redefine any functions whose behavior needs to change before I
export it. Is that possible?

Todd