My best guess is that the software will be modified so that the engine runs the same all the time. Then either its' will mileage will go down or its' emissions will go up. VW is not doubt opt for a decrease in mileage.
The un-faked emissions are something like 40x passable. Opting for decrease in mileage is probably impossible as gas burned per mile means far more gas burned and thus greater emissions (handled by an already overloaded emissions control system). They MUST get the emissions down; the only sufficient way is to decrease performance.
One big problem with them de-tuning the engine for emissions purposes. They publish a advertised horsepower and torque for the engines.
In the olden days, those numbers were just made up out of thin air and could be significantly more or less than actual. GM was famous for hand building engines out of the best parts and then selling on those numbers.
Ford found out that you can not do that anymore some years ago when some Mustang owners put their new cars on the dyno and it failed to make advertised power.