My validation rule says that the status of my object can only go from the "Prospect" status to the "Tentative" or "Lost" status values. How can I create a formula to reflect this?

 My validation rule says that the status of my object can only go from the "Prospect" status to the "Tentative" or "Lost" status values. How can I create a formula to reflect this?

 My validation rule says that the status of my object can only go from the "Prospect" status to the "Tentative" or "Lost" status values. How can I create a formula to reflect this?