Fields that customers can edit only on ticket creation

Priority fields are something we use to help automate the rest of our processes. Email alerts, escalation procedures and things of the nature. However if a customer can edit the priority field after the ticket has been created because they get impatient it can cause some serious issues down the line. An easy fix would be a check box in the ticket field editor under the customer options that simply reads as  “customer can edit upon creation only” this would allow customers to set priority when the issue is created, but not edit the field after the ticket is created.