For Visual Studio 2010 Web based application we have Config Transformation features by which we can maintain multiple configuration files for different environments. But the
Install "Configuration Transform Tool" in Visual Studio from Marketplace and restart VS. You will be able to see menu preview transform for app.config as well.
https://marketplace.visualstudio.com/items?itemName=GolanAvraham.ConfigurationTransform
I wrote nice extension to automate app.config transformation like the one built in Web Application Project Configuration Transform
The biggest advantage of this extension is that you don’t need to install it on all build machines