From the outside, the upgrades have the biggest effect on the front and rear fascia.
These photos only let us see the exterior, and the changes appear fairly minor. The C43 features a revised front fascia with new strakes in the corner intakes. The same pieces on the C63 appear slightly larger than on the current vehicle. Both of these test mules have concealed grilles, and it seems likely that the refresh would include a switch to vertical slats like other current AMG models. Updated LED headlights also look like the parts on the latest E-Class.
Mercedes conceals the rear bumper on both vehicles, suggesting updates for them. The C43 here wears a pair of circular exhaust pipes coming from each corner, but they appear to be provisional parts for testing. The ones on the C63 have a squared-off shape, which is likely the final design.
Previous spy photos of the refreshed C-Class reveals that the model has a new infotainment system. A touchpad on the center console now acts as the controller. There’s also a revised steering wheel.
Gallery: Mercedes-AMG C63 Coupe facelift spy photos
Powertrain upgrades are a mystery at the moment. The C43 could conceivably move to an inline six, but the 4.0-liter biturbo V8 would likely remain under the C63's hood. Rumors even suggest the possibility of a hardcore C63 R that would act as the hardcore range topper in the lineup.
Look for the refreshed C-Class to be on sale next year, but the date for the actual reveal is still a mystery. There’s even a possibility that Mercedes could release the full details about the updated sedan before the end of 2017.