A nature change would change stats, but not all of them, just the affected ones changing. It doesn't really matter if it's level 100.
So say you wanted to change from, say, Lax (+Def, -SDef) to Adamant (+Atk, -SAtk), those would be the only stats changing. Defense and Special Attack would go down, while Special Defense and Attack would go up. Speed was not touched in this scenario, so it will remain the same.