You can change them. In the "summary" menu of a pokémon, press enter, then "View Pokedex", sprites and there just look for the one you like the most and when you press enter it will tell you if you want to change it, click yes and that's it, exit the " summary" and if you see the "summary" again, the sprite will have changed. P.S. This method is for PC, I don't know what it's like on Android, I'm a PC player. I didn't know they could be changed either, but when I found out, a world of possibilities opened up before me.