Testing the change I made was fairly straightforward and is a good example of a small, but complete, test matrix. The feature behaved differently if a registry key was set AND the number of pages was above 10, so the matrix was pretty small:
And that is it. There was nothing special about other test cases - international considerations don't matter, performance was unchanged and so on. The only testing built into this would have to do with the equivalence class of the Registry Key being "OFF" is the same as the registry key not being set at all. If the key does not exist - that is the case right after you install and you never change the setting in options - OneNote will behave the same as it did before the service pack if the number of pages is less than 10, and ask otherwise.
Testing took very little time. Most of this was automated so I just ran those tests and managed to get this fix done fairly quickly.
Now I am looking at the feedback from the users that want even more flexibility than what we have added so far. Feel free to keep the conversation going!
Questions, comments, concerns and criticisms always welcome, John