Luckily, it's all simple if you treat each wire on it's own. CANBUS cars would be a nightmare to convert.
The difficult thing is reverse-engineering the switches so they look retro. BMW switches are not simple affairs. Very few of them can be replaced with a simple toggle switch.
Fortunately, the wiring diagrams give you a very good idea of what is inside each switch, and what you need to do to make a simple toggle switch do some quite complicated things.
Believe it, or not, the hardest bit is the heater controls. I really wish the heater controls wer all electronic, then my fascia would be completely different. As it is the whole fascia is based around the position and look of those three dials.