There is no way to take an existing template and apply it to an existing page
"out of the box."
There is a powertoy which will let you do this, but it will require a small
amount of work.
Here's what to do:
1. Download and install the powertoy (link is below)
2. Create a new page in your notebook with the template you want to use
3. Use the powertoy to create a new (external to onenote) template from the
current page
4. Now you can apply that template to existing pages.
Pay attention to the "Replace Content" directions for the addin.
Link to the powertoy:
http://blogs.msdn.com/johnguin/archive/2007/12/13/template-manager-powertoy-for-onenote-2007.aspx
