As I just completed modifying my dash from the single DIN configuration to double DIN, I thought I'd document here some of the details required for the modification.
A couple words of warning to begin with. If you are paranoid on minor scratches on your interior, please don't do this. Most likely you will end up with one or few. Likewise, if you are afraid of applying force when pulling parts out, please don't do this. Some of the parts (the aircon unit for example) are fitted quite tightly, and need a certain amount of force to remove.
If you are still ready to go forward, here are the parts you will need:
NOTE: All partnumbers are for a left-hand driven vehicle. They should be the same for RHD, but until someone actually confirms this, please don't blame me if these don't work for you.
8P0 858 005 A (double DIN mounting cage)
8P0 863 321 A (frame for buttons)
8P0 927 134 C (esp button - center)
8P0 941 515 C (blind button - left)
8P0 941 516 C (blind button - right)
8P0 941 509 A (hazard button)
8P0 941 561 E (storage on console - "coin holder")
8P0 864 348 (bracket from trim)
8P0 864 147 A (covering)
If you have a switch for the passenger airbag in your glovebox, replace the "blind button - right" with this:
8P0 919 234 A (button with airbag warning light - right)
Note that with the double DIN configuration you have much less storage space below the aircon unit than with single DIN. If you keep the ashtray, there is hardly any storage space left at all. If you don't need the ashtray, you can replace it with a tray which provides some more storage space. The partnumber for the tray should be 8P0 864 138 A (unconfirmed).
Also, if you have the interior light package with the nice red light on the storage space, you will loose this after the modification. I atleast didn't find a way to keep the light - it gets blocked by the new coverings.
And then the aircon control unit. I decided against purchasing a new which would have fit directly given the prices for these units. Instead, I modified the old unit by slowly cutting some 2-3mm from both sides and checking to see if it fit at regular intervals. If you do it carefully, once ready it will be very hard to see that it has been modified. I don't know the partnumber for the double DIN aircon unit - if anyone does, please post here.
All in all it took me some 5-6 hours to complete the modification. So reserve plenty of time and take it slowly. Feel free to ask if you have any questions.