When you make changes in a work template, to ensure that it flows through to any work items created off that work template, you will need to reset the work to the template.
To do this, go into that work item and select the options menu (...) from the work header. You will then see the option to reset your work, as seen below:
You will then have the option to reset the work to the previous work item in the schedule, or to reset the work item to the template:
When you reset your work, any progress that was made in the tasks section will be reset completely.