Interesting idea, but I think there's a bit more to it than that.
The all-in-one graphics card coolers that AC produce are designed to cool specific places on your card and there's no guarantee that the 7900 will have exactly the same layout as your 6800.
Also, the blocks are designed to fit around (miss) certain protruding onboard components such as capacitors, etc... once again the two cards would have to be near identical in topside layout for your proposal to work.
If you choose to give it a go (I wouldn't) - good luck with it and let us know how you get on