[SalesForce] Time based workflow scheduling does not get changed even after dependent date changes

nivyaj, I posted an answer to the same (or very similar) question that another admin found helpful: When does the time-based workflow put records in it's queue?

The possible solution is:

1. Create a checkbox field on your target object called "Retroactively Triggers Workflow?"
2. Edit your workflow with the time-dependent action to "Evaluate the rule when a record is: created, and any time it’s edited to subsequently meet criteria"
3. Edit your workflow with the time-dependent action to include an OR condition for "Retroactively Triggers Workflow?" equals True
4. Do a mass data update as others have suggested to set "Retroactively Triggers Workflow?" to true for the records that need to be retroactively processed

After the workflow actions have fired, you can delete the temporary checkbox field.

The way you need to approach this is make it so that the workflow always fires, but if the Case is not overdue at the time the Time-Dependent action takes place, the email won't go out. In a deadline scenario, there are usually two factors - the deadline, and some indication of whether the task has been completed. With a Case, this is likely whether or not the Case has been Closed. So your WFR criteria could be:

NOT(ISBLANK(Due_Date__c)) && NOT(IsClosed)

Then your time dependent action would be 0 hours after Due_Date_c. If the Case is closed between the time the workflow initially fires and the Due Date, then the email won't go out. If the case is still open, then the queued email will be sent. If the Due Date is changed, then the workflow re-fires and the old email is removed from the queue and a new one based on the new date is queued up.