I did it once, had to have virtually the whole interior of the car out, replace all the speakers, add two new ones on the rear shelf, new ones in the doors, new wiring into the doors, major rewiring for the amp as all the pins are different, new wiring for new microphones in the roof. Some recoding. Was very happy with the result but decided afterwards that getting a different car would be the better way to go! Others will have a different view of course. So, yes, it can be done.
2008 A5 3.0TDI quattro Tiptronic - 2012 A5 3.0TDI quattro S-Tronic Cabriolet