How do I move the folder in which imported graphics are stored, without wrecking every file in my Bookfile?

If it's just one folder of imports at a single level of that path, don't worry about the wrecking.

When you re-open the book component files after moving the /import/ dir, and it pops the first not-found dialog, take the option to update using the new path. Frame will try that path on all subsequent not-founds. You'll need to do this only once per book component file.

This feature actually works on Windows Framemaker (and doesn't, reliably, on older Unix FMs).

If your situation is more complicated, there are other approaches (which are also more complicated).