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


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