Instead of creating a new Appointment or Post, this can be corrected through Post Profile Maintenance
Click CorePersonnel, Reference, Q - Z, Other References
Select List of Values (LOV)
Search for Post Title
Create or update Reference Fields for Post Titles
Click CorePersonnel, Maintenance, Post Management, Post Profile Maintenance
Search for Post Number
Drill down into Post Profile
Choose LOV value for Post Title
Click Save. This change will feed through to all appointments and employees that are currently set up in this post. If you don't want this to happen, the solution is to create a brand new post with the correct Post Title.