I think you're asking "what parts can be replaced without the end buyer noticing?"

You can replace almost any part on an iphone, but if it isn't an Apple part its functionality is reduced. The only people that care are people reselling iphones with substandard parts.

> The only people that care are people reselling iphones with substandard parts

So if i replace the part with perfectly good components, from other iphone, it won't complaint, right ?

...and people who want healthy competition.