Retrofitting AMI isn't difficult, I did it to my A6 Avant a few years ago.
I guess that the AMI part replaces the 6CD changer as it looks to be of a similar size. I believe there is also just an external port that you can have connected and keep the 6 cd changer.
Hardest part is sourcing the AMI, seems AUDI don't sell them any more, I could only find one seller on ebay selling a couple of Gen2 AMI boxen, they are both now gone.
Edit, scratch that, it's a CAN-bus based one, not MOST-bus.
You can either replace the changer, or split the MOST ring with a Y-Splitter and have both. To have both, you either need the glovebox designed for two units, or cut your existing one to fit.
Anyone know where I can get the software updates for the MMI from for cheap? Just had a look on the Audi site and couldn't see them (Found the latest sat nav disks for 215 euros though which they can swivel for)
I might have a software update disk lying around still.
What version is yours on?
As I replaced the CD-changer rather than keeping both, I have a power and optical splitter cable going spare.
Edit:
And the seller I got the AMI from has listed some more: http://r.ebay.com/C7Gemi
eBay Advert said:Here is a basic guide to compatibility:
A4 (B8) >2008 with 2G MMI DVD Navigation
A5 with 2G MMI DVD Navigation
Q5 with 2G MMI DVD Navigation
A6 >2005 with Colour Screen Radio or Navigation (car may need software update to enable AMI)
A8 >2003 (car may need software update to enable AMI)
Q7 with Colour Screen Radio or Navigation (car may need software update to enable AMI)