Hi

I have a 2011 A6 Avant S-line Special edition with MMI 3G Navigation which is DVD based, I've seen on ebay several multimedia units with HDD including ones that have the phone sim slot.
Does anyone know if the DVD nav to HDD nav retrofit is just a case of replacing the multimedia unit or is there more involved?

I've done some googleing but most retorfits are basic to nav where as I'm looking to go to the nav hdd to have DVD playback available.

Dave N