I have an approval workflow that assigns a task for approval.  Whenever I make a change to the workflow and publish the change, it breaks any existing tasks.  The task forms will not open.  I have to cancel all existing workflows and restart them to assign new tasks.  

Is this normal behavior or am I doing something wrong?

Also - what is the difference between the workflow setting, overwrite existing version?  Could this setting be causing my problem?

I'm a developer, but just learning Nintex workflows.