Has anyone successfully upgraded their phone?

I wonder if anyone here has managed a successful hardware upgrade of their phone?

The things that come to my mind are:

* camera (provided there's a better model with the same electrical interface)

* CPU (provided there's a higher performance one with the same pinout)

* RAM (provided it's not a PoP SoC or not otherwise integrated to the CPU)

* Flash ROM (seems straightforward, but haven't heard of that, too)

or anything else, really.

Failing that, any steps you tried to that end, but failed?

I googled around, but failed to find any quality discourse on this topic at all.