PRN reason hard coded after order entry complete

When administering a PRN medication, the PRN reason drop down list is available allowing the nurse to choose a different PRN reason. This allows the medication to be given for a reason other than what was ordered. Which I believe would be operating outside the scope of practice. We are requesting that this field in medication administration be hard coded to what the prn reason was when ordered. If it needs to be changed, then a new order should be entered.
At the very least, can programming be made that will default back to the ordered PRN reason for the subsequent administrations if it is changed previously. Currently if a nurse changes the PRN reason, then that reason is sticky for the next nurse who administers. They are not catching that the PRN reason is different and then their documentation in the flowchart differs from what is on the MAR.