On a second thought: If your customers are satisfied, they will give good reviews on the the new version and it will regain its rankings. That's a great risk for the developer, I have to admit.
This approach also forces the developer to build in a notification system to tell people the new app is out and how to find it.
I guess you could can make a small update to the 'old' app that will pop up a window notifying of the new one when the old app first starts, but that seems rather kludgy.
On a second thought: If your customers are satisfied, they will give good reviews on the the new version and it will regain its rankings. That's a great risk for the developer, I have to admit.