Would changing the sprite need a new set of animation?

I'm a beginner in Unity and I have made two simple games so my knowledge about scripting is pretty basic (like jumping, walking, and dying from colliders..). Now I'm planning to make a 2D platformer game where if my player picks up an item (backpack), then that item would stick to him (on his back). I've been looking through youtube and the forums if I can change what my sprite would look like and it can be done. However I'm visualizing the output: if I make an animation of my player without a backpack walking, then if I did change my character's sprite (now with a backpack), as soon as I start walking then the character would walk without a backpack until he stops. Is this correct or should I know something else, hence the question?

I would really appreciate it if there's a manual or a tutorial about this. I just don't know how to properly call it so maybe that's the reason why I don't find anything in my searches. And I would really be thankful if you could provide a sample script but again, just a manual or a link would be awesome. Thanks!