(RH8) How do I use Project Settings to select a CSS?

What I've done so far:

1. File > Project Settings
2. Clicked [Add...] to find the css file I want to use.
3. Selected 'Create a Reference'
4. Went on with my work (importing FrameMaker docs into the project to see what they look like).

That worked fine.

Now, however, I have had to make a change to the .css file. When I open up the project again, and import Frame docs again, the available style mappings (in Edit conversion settings for FrameMaker documents) still show the *old* css formats. Thus, I assume RH isn't looking at my current css file (I 'verified' this by renaming it temporarily without affecting RH).

So... what do I do? How do I modify the css without creating a *new* file and adding it to the Project Settings? (because then that list gets crowded with garbage)

Thanks. (I know it'll be obvious, but I can't seem to find the answer.)