I guess figuring out the best way to document changes in templates might
be a good start. Part of the problem with roundup is that it is so
flexible, and people will generally add their own customisations to
templates.

Other than generating a diff of all of the template files for each
template just before release, is there a good way to document changes in
any concise way?

Personally I think the best way to upgrade is to do a three-way merge
with the old version, new version, and the customised version of the
template... but that isn't really documentation.