how do I change a 2D Sprite Character Image/Graphic to another Sprite Character Image/Graphic with the same movements, animation,mechanim etc.

I must be confused about the question. Otherwise, wouldn't this be as simple as changing the sprite in the SpriteRenderer?

http://docs.unity3d.com/ScriptReference/SpriteRenderer-sprite.html

 [SerializeField] Sprite newSprite;
 
 GetComponent<SpriteRenderer>().sprite = newSprite;

The animation will only effect this if the animation itself changes the Sprite in SpriteRenderer. (Which can be a problem that requires a different solution. Most likely not have any SpriteRenderer data (change) in the animation timeline itself).

You can also have animations on parents of sprites. What I do for all of my game entities is have a hierarchy like this:

 -NPC (A root gameObject that holds its children. May also hold AI scripts, character game Statistics (Strength/Health/Stamina/etc.)
 --Graphic (SpriteRenderer, Layer adjusting script, etc.)
 --Physics (BoxCollider, input interaction/response scripts, anything that deals with collision detection)

For complex graphics, I do the same thing.

  -NPC
  --GraphicParent
  ---Head
  ----Graphic
  ---Torso
  ----Graphic
  ---Larm
  ----Graphic
  ---Rarm
  ----Graphic
  ---Lleg
  ----Graphic
  ---Rleg
  ----Graphic

By animating the parent fo the Graphic (Head, Torso, Larm, Rarm, etc.) you don't ever move the graphic. So you can move the parent of the graphic.

I'm not thinking about this much, so it may not at all matter logically if you do this. (Is there even a benefit?) I mostly do this to organize my components in a way I can understand and interact with easier in the UnityEditor Hierarchy.