If I change the Date Sent value for a letter, must I also change the Due Date for the action item associated with the letter?

No. Unless the action item that is associated with the letter has already been completed, when you change the Date Sent value for the letter, Oracle Argus Safety also updates:

• The Due Date for the associated action item, to match the new Date Sent value.
• The number of days for the action item that is specified in the letter configuration.