Hello Rainer
By issuing the no mls qos command, you will not disrupt the functionality of the device. It will continue to forward traffic normally, but it will simply stop enforcing any QoS mechanisms that may be configured. Even so, it is still a good idea to implement this during a maintenance window.
Keep in mind that although the command itself will not stop the device from continuing to function, it will suddenly change the way that traffic flows on the network. This can have some unpredictable results like unexpected congestion and bottlenecks at various locations. When you do implement this change, make sure that you are on hand to monitor the behaviour of the network and to ensure that it indeed is functioning within normal parameters. You should monitor the network during the next few days as well, especially during peak hours to verify that you are getting the expected results from your change.
Let us know how it goes!
I hope this has been helpful!
Laz