Volkswagen (VW) plans to refit up to 11 million diesel vehicles with proper emission-control systems that will bring them up to pollution standards. However, these fixes could actually cause the vehicles to become less powerful and less fuel efficient, experts said on Tuesday.

VW's new CEO Matthias Mueller said diesel cars that need to be refitted will have changes made to ensure they are not polluting, Reuters reports. VW did not say exactly what would be required to refit the vehicles, but some analysts are predicting the fixes could cost over $6.5 billion.

Mueller informed 1,000 managers from VW of the plan to fix the vehicles Monday at the company's headquarters in Wolfsburg, Germany. Over the weekend, VW created a special team that will inform owners of the vehicles on how to proceed to get their vehicles fixed.

Some experts say that VW will only be required to change the software inside these affected vehicles. Others say there will be two different solutions required for two different sets of emission-control systems installed on almost 500,000 U.S. diesel cars from the model years of 2009-2015.

VW was found guilty of using software that limited pollution, but that only turned on when a vehicle was going through emissions testing. During normal driving, the cars polluted from 10 to 40 times more than the Environmental Protection Agency (EPA) allows. VW admitted that they were trying to cheat emissions controls by using this software.

The cheating system that VW used was known as a "defeat device" and it allowed these cars to pass emissions tests, making them legal for driving.

VW vehicles had "lean NOx traps" designed to reduce nitrogen oxides in engine exhaust. Nitrogen oxide emissions have been linked with smog, acid rain and lung cancer.

Experts like automotive consultant Sandy Munro say vehicles with devices to control nitrogen oxide emissions typically have lower performance and diminished fuel economy. VW was able to use software to turn these devices off during normal driving to give the diesel vehicles better performance and gas mileage.

Analyst Kevin Riddell of LMC Automotive says fixing the software on these vehicles will result in lower performance and worse gas mileage. He also says that the cars' values could go down, and it will be harder for owners to re-sell.