IDVD can update automatically but you need to do some steps manually and build the DVD manually in iDVD.
1. In iMovie share the movie to the Media Browser.
2. Open iDVD and choose a theme and click on Project > add Movie...
3. Navigate to the movie in the Media Browser in iDVD and drag it onto the Add Movie button.
4. Now in iMovie if you make any changes to the project you will need to save it to the Media Browser again.
5. Any changes in iDVD will not show yet. First you must quit and then restart iDVD. Now iDVD will be updated with the new edit of your movie project.
iMovie must share the movie in some way so that iDVD can read and recognise the changes you have made.
Hope that helps,