Scott:
One of the things that I've been able to do successfully is save a single FrameMaker chapter file as a MIF, edit that MIF in a text editor, and use that editor's Find-Replace to change certain aspects of paragraph styles.
For example, if the problem is to globally change several paragraph styles from "Font_A" to "Font_B", I use the Paragraph Designer to update one paragraph style with Font_B before saving the file as a MIF. This enables me to get the correct in-file syntax for the new font's name, then use Find-Replace to update all instances.
From there, I open the MIF in FrameMaker, save it as an .fm file, then import that .fm file's styles into any other chapter files that need updating.
This works for me because (a) I usually only update a template set once at the start of a project, and (b) getting one chapter file converted makes it trivial to convert any others that need the updates. After that I seldom need to do any wholesale changes that would benefit from a built-in templates modification tool...
Cheers & hope this helps,
Riley