A template is not just a set of initial defaults for a report - it is the basis for that report. So if a template is updated, completed reports that were based on that template will also change.
Note: Note: Report Design Wizard templates have the opposite behavior; when an RDW template is changed, none of the reports based on that template will be automatically be updated with those changes. Instead, you will have to edit each report, apply the new template, and save it.