Really useful insights from NN/g: Features meant to increase user efficiency by reducing steps can end up hurting users if they do not conform to existing mental models and expectations based on past experiences.
I think this is one of those usability issues that is greatly enhance by ageing.
So even if the best practice about usability says you should change something you need to figure into your calculations the mental angst it will cause the older users.
It is a bit like my local Waitrose (UK supermarket) that recently had a complete reconfiguration of its layout to make it much more logical. Months afterwards you still hear older customers moaning that they cannot find anything. I am sure that it has had an impact on revenue.
This doesn't meant you should never change but you do need to take account of learned behaviour that you are about to make redundant.
Image via flickr