At lv 100 the nature of Pokemons still matters?

Question

The nature of a Pokemon really matters ?
If I change the nature of my Pokemon will I lose my EV's IV's ?
How can I change the nature in SW/SH ?

Answer 1

First Question: Natures don't matter in game, but in competitive battles they can make a big difference to outspeeding other Pokemon, getting an extra bit of damage off on an opposing Pokemon, or taking an attack better.

Second Question: You won't lose any EVs/IVs when changing a nature, since the only way to change a Pokemon's nature is through a Nature Mint, which doesn't affect EVs or IVs.

Third Question: As said before, the only way to change a Pokemon's nature is through a Nature Mint, which can be obtained in three ways,
1. Battle Tower Singles/Doubles by reaching certain ranks
2. Online Ranked Battles by reaching certain ranks
3. Buying a Battle Mint for 50 BP
Using a Nature Mint will change the selected Pokemon's nature to the type of mint.

Hope this helped!
Source: Experience