Lol

Can you tell me how Luther starting his own church changed anything for the Catholic Church which had already existed for close to 1500 years ?

Well the Catholic Church started the Catholic Reformation, aka the counter reformation, which changed a bunch of things in the Catholic Church like the sale of indulgences and a few other thing, which eventually led the the signing of the Augsburg confessions and multiple councils, like the council of Trent

Yes those were done in defence of the Church. Perhaps it took someone like Luther to point out the problems but his approach was certainly not one of wanting to make the church better, he wanted a whole new church of which he was the leader.

Originally, he wanted to make the church better, but when the Catholic Church didn’t listen and tried to kill him, he created a new church and split.