Any workflow design changes can be pushed to the sleeping process instance(s) as shown below. 

On the Process Instances page select the process from the table and click on the Utils - Update - Design menu option.

An Update Design popup window is displayed for confirmation. The application collates all design changes to push to the sleeping process instance (related to the definition). Click OK to proceed.  

The design changes are updated successfully with an alert notification as shown below. 

Note: The values passed to Variables/Globals to a process instance through API are retained by default, whether the values are correct or not.