How can I automate the conversion of a document created with a given
template and style and convert it to a different template which has
different styles, without manually reformatting each paragraph?

If I understand it correctly, the “Automatically Update” option under
Format/Style/Modify Style dialog box, allows you to update an old
document to new styles, but the old document must be the same template.

My problem is I have mannuals created with different templates and
styles and I want to convert them to the same template with the same
type of styles.

Thanks in advance,
Ron