(Sorry about moving your post but wanted to make it easier for others to see this info)
Unfortunately, derived fields are one way mirrors. You cannot edit the information in a derived field. However, you could use it to transfer information and then script to rebuild the information i.e. ECO to ECR, but this would be way more complex then needed.
The best solution for this would be to have a single select field for ECO or ECR, with a separate filed for the number. Then what you could do is use the derived field for the number only and allow the Item descriptor to combine field 1 and field 2 for you with would in turn give you a ECO or ECR number for the item in question.
This should help you achieve the desired effect for what you’re looking for.
Joe Piggee Support Specialist Product Support Autodesk, Inc.