An RDW template is only a set of initial defaults for a report, so if a template is updated, completed reports that were based on that template will not be affected; there is no connection between the template and the report once the report is saved. If you want to update an RDW-based report to reflect template changes, you can edit the report with Report Design Wizard, make any necessary selections, and re-save it.
Note: Pentaho Interactive Reports templates have the opposite behavior because the report links itself to the template; when an Interactive Reports template is changed, all reports based on that template will automatically inherit the updated template.