Launch config name is not required for update

UpdateAutoScalingGroup doesn't require LaunchConfigurationName parameter to be sent.
See http://docs.aws.amazon.com/AutoScaling/2011-01-01/APIReference/API_UpdateAutoScalingGroup.html

In cases where you want to update a group (for example change min size) it is unnecessary and cumbersome to look up the LaunchConfigurationName each time.