We've added new functionality when working with Route Groups, allowing users to re-optimize one or more individual routes contained within a group, while maintaining the initial optimization of other routes within the same group.
From the All Routes view of any Route Group, use the checkboxes to select the route(s) requiring re-optimization; from the menu that appears at the bottom of the page, select Optimize route(s) to re-optimize the route(s) that have been selected.