I think you can only change a shortcut Icon's image. Changing the actual Excel App Icon is another story. I've been there!
Right Clicking on a shortcut Icon will give you the option of finding the original application or changing the Icon image, however that image won't follow the Icon from computer to computer. You would need to do this routine to each shortcut Icon, on each computer.
There are other ways, but I don't know how involved you want to get with this.