What is the problem of Updating Unity 4 to Unity 5

I updated my Unity from 4 to Unity 5. Should I change my previous scripts manually?

In Most Of Them The API Updater Will Do The Job, But It Might Not Update Or Be Able To Fix Every Scripts You Have! But I’m Wondering How People Are Still In Unity 4 :D.
BTW You Can Update Some Of The Words And Lines Like This:
-RIGIDBODY COMPONENT:

rigidbody.mass //UNITY 4   
GetComponent.<Rigidbody>().mass //UNITY 5 JavaScript
GetComponent<Rigidbody>().mass //UNITY 5 C#

-RENDERER COMPONENT:

renderer.material //UNITY 4
GetComponent.<Renderer>().material //UNITY 5 JavaScript
GetComponent<Renderer>().material //UNITY 5 C#

etc…