Of course they different. That's the whole idea.
I've seen A4 B6 completely remade into the B7, from outside. Owner was prepared to modify he's B6 in such a way so the B7 pieces will fit on. You could probably get the new B7 for the time, effort and money put together but you kill all the fun from it.
Also, seen A4 B7 with a custom front bumper adapted half from B7 and half from B8. He cut diagonally in half both bumper and matched up B7 upper part to match with wings, bonnet and lamps, with B8 bottom part. He achieved excellent result. How chuffed this guy was. There is no car like his in the entire world.
Question is: are you prepared to go with it, custom make it/tweak it a little bit or just want simple bolt-on swap?
If rear diffusor from C7 A6 would need some minor modification to fit onto my C6, I would do it.
Replacing the whole front, I wouldn't. Unless had the money and time.