Not sure I agree with this at all. Its hard to loose people, but we all change in one way or another and its not hard to remember where we have been or who we used to be. In fact its important that we dont.
Changes are good...weather its bad change or good change...it teaches us various lessons about ourselves...and yes as much as it is hard to watch people change...it is not hard to remember who they use to be if you let yourself only remember the good things and not the bad things. When you like the way you use to be and you changed ... you will remember as it will be an important aspect of wanting to be who you want to be. I know that from experience. And if you allow people to change the way you want to be then they are not worth spending time to change for.