The moment the change happens, Yhwach is already erased.
He can’t do anything to prevent changes made in the past. There is no present or future changing, everything after the point in time Zeref travels to is erased to begin anew.
Yhwach has never countered a timeline being erased. And he only exists in one place. That’s why killing him is even possible. He can change the future to what he likes, he doesn’t exist in them all at once, wanker
Juha sees all timelines, all infinite timelines, it is impossible to change a timeline without him knowing, you seem to really don't understand this simple fact, but I wont expect much understanding and intelligence from someone who is clearly a toxic delusional Fairy Fail fan.