Here's my understanding, such as it is:
The 'Symphony' versions of the later head units have CD changers, the 'Concert' versions are single-disc... that's basically the difference.
The Concert/Symphony 2
+ have NO cassette player, and a larger screen in its place.
The 2+ (both symphony and concert) also have a quadlock connector on the rear in place of the separate connections, as well as a twin-Fakra for he antenna, so I suspect that you'll need a harness to convert from an earlier (concert or concert 2) to either a concert 2+ or symphony 2+.
The newer 2+ units also have to be VAG-COM coded for passive or active rear speakers, etc.
I have a mint condition one here on the bench, but it's a Concert 2+, not a symphony, or I'd happily sell it to you. It can be coded for USA or European stations.
You'd obviously also need to do the double-din conversion. (I'm assuming that you DON'T have a double-din Concert 2+ at the moment is that right or am I misreading?)
{EDIT]Okay, wait. I THINK I get it now... you have the 2+ already, you just want to move from a concert to a symphony... -Yours looks like this, -right?
(That's a TT version, but basically the same apart from the front panel shape)
Keith