Smooth UI sprite animation.

I’m using the animation feature on a button, I want to make the highlighted animation a animation where the button sprite smoothly fades into another sprite. But when I run the animation the button sprite stays the same and when it reaches the animation key where it needs to change the sprite, it simply changes to the other sprite without a smooth effect or anythinh. Does anyone know how to make a smooth animation?

I have read on stackoverflow that this effect can be achieved trough code… is the anybody who has experience with this?