I did the double in conversion a little while back. I bought a double DIN climate control unit but ended up just using it to make a template and cut the original one down to size. The double DIN climate units don't have the metal bottom edge that the single DIN ones have, and this metal bit is a pain to cut. The whole lot needs filing/sanding down smooth, especially the metal edge or you'll scratch your centre console when reinstalling it.
The centre console has to come out as that needs to be cut so it aligns to the double DIN cage correctly. There's a comparison of the single and double DIN consoles here:
A4 Centre Console DIN variations | Matt Easom
Also the ashtrays are different, and I never actually seen this documented anywhere before. The double DIN ashtray has a vertical folding flap that covers the 10mm gap between the base of the climate unit and the back of the ashtray.
For the single DIN configuration Audi put a plastic base that fits behind the ashtray to create a little cubby hole under your climate unit. You can either cut this plastic base down to fit what will be your DIY double DIN console, or throw it somewhere in the shed and find a double DIN ashtray.
I didn't go for an RNS-E, I just put a concert III unit in but I found it took a LOT of force to get it to slide down the slots on the cage. I had thought I had cables in the way behind, but no, there were small metal guides on the side of the radio that seemed to be a little fat for the slots in the cage. Nothing a considerable amount of brute force couldn't fix.
Couldn't be happier with the result though...