iPhone with new battery vs old battery. What is going to happen?

Before months Apple has official say that downgrade a lot of iPhones because of battery drain faster than usually battery.So this upset a lot of users around the world and the brand go and answer that will decrease the value of  battery replacement. And they did it .So a channel on youtube try to test if APPLE really say the truth and whats is the different.

https://www.youtube.com/watch?time_continue=76&v=qCD_psdhCuU