If you get Audi OEM xenon housings, relatively simple.
allroad xenon housings will also fit perfectly in the A6 (I have these in mine). It's simply a question of removing the front bumper cover, so that the ballasts strapped to the bottom of the housings will clear, slide into place, bolt down, and plug the wiring harness into the connector.
The auto levelers will not work, since there is no rear axle sensor on OEM non-xenon cars, but that is a minor matter. Other than that, it is pretty much plug n play, and using the manual adjusters to aim the lights.