Can you put an A11 chip into an iPhone 6 logic board

Hello Alex, unfortunately even identical CPU replacements in iPhones are beyond even the best micro-solderer's ability. You could not even swap an A8 for another A8 taken from a different iPhone 6 motherboard. This is because the CPU, Modem, Flash memory, and eeprom chips are all paired to one another. So if i wanted to move a good CPU to another motherboard, I would actually have to move over all 4 BGA chips without damaging even one.

As to upgrades, typically the CPU and modem chips get redesigned/ upgraded each year, meaning the new motherboard is designed to fit a different package design. So besides all the issues of programming, the newer components would not fit onto older board's solder joints.