In this post i'm going to walk you through some of the changes made by Audi for their lower downforce/drag configuration, please remember that the images are of a much higher resolution than those visible in the post, clicking on them will take you to the full res version.
Firstly I'd just like to say that the level of detail changes on the R18 between the two variants is staggering, they are basically two different cars.
The change in shape has also led to other changes, including a revised set of headlights (blue), the shaping of which is led by the change in ductwork. Rather than the singular large inboard duct the design team have implemented two smaller ducts, which continue to serve the purpose of controlling how the air flows around the front wheel / tyre toward the sidepods turning vanes. The quantity of dive planes (red) were shown as reduced from two per arch to one too, with the angle of attack also being reduced, clearly another attempt at reducing drag in the studio released image. However, at Spa (Below) Audi ran with two dive planes, albeit with the upper one reduced in length owing to the ducts interaction with it.
The change of arch shape also means a shallower transition on the upper surface of the arch with the team moving and reshaping the bulge that sat alongside the mirror in the HD configuration, but houses the mirror in the LD configuration. This also leads to a small change in the length of the sidepod (purple) as more of the surface becomes exposed.
Rear end images of the R18 LD configuration are a little thin on the ground at this stage but once I get hold of some I may update this post.
The changes made by Audi ahead of Le Mans are about reducing drag, however, they weren't particularly slow through the tighter, twistier middle sector in Spa and so clearly weren't giving up huge chunks of downforce, relative to the opposition. Efficiency is the name of the game at the end of the day and so it appears they have been able to retain similar levels of downforce for less drag, making the package ideal for Le Mans.