Sharepoint - "Pause until Date" workflows unable to handle changing dates?

If this is a 2013 workflow you could use the new loop feature to periodically check if a date is within range.

Something like this (short pause, for testing):

For 2010 workflows, I recommend using a parallel block with two steps.

The first step starts off as "Wait for field change" in your Due Date field. If it has changed, then you can restart the workflow from the top (I use a hidden boolean field as a flag).

The second step would have your "Pause until" action. In this block, add your condition to check whether your task has already been set. If it hasn't, continue to send your reminder.

By using parallel block it can achieve like this.

The variable EndParallel is set as the CompleteCondition property of the parallel block (Right click on the parallel block and go to Advanced Properties ). When the variable is set to true the two steps in the parallel block got cancelled and will come to transition stage.