If certain effects have been applied to the text in 2007, you won't be able
to edit them in 2003. Or if the tree was created using the new 2007
SmartArt, you won't be able to edit it in 2003. You could download and
install the 60-day free Office 2007 trial version, but that seems like
overkill just for a family tree -- and installation problems related to
Outlook 2007 and 2003 seem to be fairly common.

If I were in your position, I'd use the template as a guideline and insert
my own textboxes/autoshapes to mimic the family tree and create my own that
way.