The worst thing that could happen when creating a long iDVD project was to realize, after burning, that there was a typo in the credits or a sound glitch that you had missed in editing. Bad enough to go back and re-edit the footage, and re-output to an MOV file, that's some time and effort. But the worst part was that when you re-opened your iDVD project, iDVD would throw up an error - "Modified Asset Warning."
The only thing iDVD would fail on at that point would be to mess up all your chapter buttons. They would still be there, with their proper names, but they would all point to chapter one. This was unavoidable, even if you had named the file the same thing, all the chapter markers were in the same place, etc etc.
Since renaming chapter buttons and selecting thumbnails is so clunky in iDVD (my projects often have 40 chapters per disk!) it's worth finding a way to avoid this error if you ever need to re-export a video file with minor changes.
I finally figured out that the reason it fails to accept the file is that the TIME STAMP has changed! This is truly the mark of an amateur program - good as it is. It assumes that if the file has been modified since you last opened iDVD, then there must be a mistake, and it doesn't even bother checking the length, the file size, the placement of chapter markers - anything!
Notice the "modified asset warning" error message - click through to see an example - complains about the date stamp! It's easy to miss if you don't hit the dropdown triangle - the first time I've ever seen such a drop-down in an error message. No wonder I never noticed it before.