Convert your OneNote Notebooks to 2007 Format
If you open a notebook from OneNote 2010 in OneNote 2007, you may see this warning informing you that the notebook was created in a newer version of OneNote and cannot be opened.
To make your 2010 notebooks compatible with OneNote 2007, you need to convert them inside OneNote 2010. In OneNote 2010, open the File menu; this should open to the Info tab by default. Select the Settings button beside the notebook you want to use in OneNote 2007, and select Properties.
In the properties dialog, click “Convert to 2007”.
You may see a warning that some formatting, content, and history that is incompatible with OneNote 2007 will be removed. Click Ok to continue.
OneNote will automatically convert everything in this notebook to 2007 format. If your notebook is very large, this may take a few minutes.
Once the conversion is completed, you can re-open the properties dialog to see the change. The format is listed as OneNote 2007 format, and you have the option to convert to 2010. Your 2007 formatted notebook is still fully usable in OneNote 2010, but you may not be able to use some of the newer features in it.
Now that your notebook is in 2007 format, you can share it with OneNote 2007 users. Here’s our notebook, the OneNote 2010 guide, open in OneNote 2007 after the conversion.
from http://www.howtogeek.com/howto/15558/share-onenote-2010-notebooks-with-onenote-2007/ Answered Jun, 26 2011