We have the confluence suite.
Using this, is it possible to produce a report highlighting what classes have changed?
JIRA items linked to GreenHopper via SVN+JIRA references is exactly the type of information I need.
Does such a report template exist?
The best way to get this out of the box is to get FishEye, Atlassian's repository browser tool. With the latest versions you can embed FishEye content in a JIRA dashboard and get the reports you need without having to switch to FishEye.
As Francis mentioned already, post back here if you have more detailed questions.