I have the same question. BGA resoldering isn’t a concern, there’s another tricky thing - EFI and configurational smds on the board. For instance, if you want to upgrade RAM it’s tottaly doable except you MUST KNOW which smd configuration is needed for your new setup, so you have to change it in a very special way. Is that the same for CPU? Or the work is to simply resolder the new chip?
I got it working finally. I turned out to be not as simple! After I did a Lightning replacement some time ago I didn’t notice the right placement for a coaxial cable laying in the bottom of assembly. I stuffed it under the speakers and THAT WAS WRONG MOVE, as they lifted a bit. So when I glued the screen it had a stress on the bottom. I didn’t even notice it as it was laying flush. But after a drop something interrupted there and it touch started glitching. I suspect it needs a replacement anyway, but for now it works.
A little note (I think if we’ll find the solution it would be helpful for others so excuse my tediousness :P) To check the screen I try to type a text. It’s getting harder as a half of the screen glitches now, and what I found - quite often when the key does not print a character I HEAR THE SOUND OF THE KEY PRESSED! So it actually registers the touch. Also, sometimes touch gets a little hysteria like I use second finger somewhere. So now I suspect like it thinks like there’s a phantom touch! WTF? How to diagnose this? What could be the cause? PS. I use http://spark.attrakt.se/ for the tests. JB and tweaks are disabled