如何在Wix中读取app.config值并设置属性

I have a .NET program that I've written a Wix installer for. I want to be able to update from a pre-Wix version. To do this, I'd like to read from the installed program's app.config file. This will save the user from having to manually retype in all of these values. Is there anyway to do this? Thanks!I have a .NET program that I've written a Wix i