Does it not work for your circumstances to import the styles from the new template into the old documents, by attaching the new template and temporarily checking the Update Styles box in the dialogue? If that were adequate, someone might be able to provide you a macro to do it, if you simply filled in the template location; or, recording the macro might do work. But perhaps the style names change and won't work for you this way. Just a thought.

Best,
Ulo