Info about porting roms. topic

Hello, I wanted to know that how to port the more high versions rom like 4.2.2+ to our 4.1.2 based Nokia XL? I'm seeing guides to port roms and learning a bit from them. But they say that it requires some special things if you want to port 4.2.2+ on your device (nokia xl) which is based on the 4.1.2.

Thanks :)!

Sent from my Nokia_XL