OK - a lot still depends on what replacement unit you are planning, but basically -
On the back of the Audi unit you will most likely find 2 standard ISO connectors - one for power and related stuff, and one for speakers. So far so good as pretty much any head unit made in the last 20 years either uses these or comes with an adapter.
You can get a fascia plate for about a fiver to actually physically fit a standard single Din unit.
Now the fun starts - The OEM unit uses canbus to to receive data on the status of the ignition, instrument panel illumination etc. If you wish the unit to be switched on/off with the key, illumination to be controlled by the dash control, speed sensitive volume etc, you either need a unit that has canbus (rare) or an interface and adapter loom.
Another frequent issue reported on here is that only the front speakers work. Even the non-Bose system uses a separate rear amplifier - the two rear speakers and the sub woofer in the boot are connected to this and signals to it come from the mini-iso connector on the back of the unit. This means the rear speaker outputs from a standard head unit are redundant and aren't wired. You will need to wire the pre-amp or line level out via a suitable adapter to the rear amp - or a re-wire of the rear.
All of this assumes you don't have factory bluetooth or volume etc on your steering wheel - if you do, it gets a bit trickier still.
