OK. I think it might change but under these conditions. If your charizard who is level 100 has a certain amount of EV, it sticks with that.
I think if you use the reset bag on it, it allows you to rebuild up your Pokemon. By using a reset bag - I might be wrong but you could go back to level 1. If not, then it's something else.
The reset bag allows you to re-control your Pokemon. For example, several bags in super training such that lets you increase attack, defense, speed etc. helps you boost these stats. When you reach level 100, BOOM!, no more stat changes. This way, you can decide to train it and focus more on attack, defense etc. - whatever you want. It might result in the same stats, depends on what you do with it.
Hope I helped! Please like if it was helpful :)