If I had seen how dead this forum was, I would not have bothered. Anyway it looks like OneNote preserves the format the notebook was origianally created in, which can be seen in the properties dialog of each notebook (right-click the notebook title, select properties). In this situation notebooks created under 2007 showed "2007 format" along with an option to convert it to 2013 format, and notebooks created under 2013 showed "2013 format" along with an option to convert to 2007 format (with possible loss of features).

The 2013 notebooks I was dealing with were apparently not using any 2013 specific features since they converted without issue, and once the OneNote Notebooks folder was copied to the new PC, there were no obvious changes in appearance of any of the notebook contents.