What is the required code for switching from FPs view to TPs view while having TPs view smooth?

Add this script to an empty GameObject and assign the cameras in the editor.

 var cam01 : GameObject; // first person 
 var cam02 : GameObject; // third person 
 
 //start with first person active
 function Start() {
    cam01.gameObject.active = true; 
    cam02.gameObject.active = false; 
 }
 
 //check for the return keyDown
 function Update() {
 
  if (Input.GetKeyDown ("return")) {
 
 cam01.gameObject.active = false; 
 cam02.gameObject.active = true; 
 
 }
 
 }

If you want to toggle between cameras using the same key you'll need to use an if statement and a boolean to check which camera is active.