The only time a Pokemon will change is if you reset your game before you encounter it. This doesn't work with eggs but does for over-world Pokemon such as Ho-oh, Lugia, Dialga, Palkia, Giratina etc. If you knock it out then it will stay exactly the same as when you first met it, you just need to beat the E4 and go back. This is because the Pokemon's statistics (i.e. IVs, Nature, Shininess, PID) are generated when you interact with the Pokemon for the first time, they are then saved to the Pokemon so any future encounters will be the same.