I've been working on a Windows Phone 8 app, which I now want to change to be Windows Phone 8.1. How can I do this?
Should I create a new Windows Phone 8.1 app and copy all my existing files into it? Or is there an automated way for me to upgrade the existing solution to 8.1?
There are two ways to do that:
In every case the VS should ask if you are sure to do that - it's one-way operation, so it's worth to make a backup of your solution. As WP8.1 has backward compability, in most cases there shouldn't be any problems.
Some clarification for the followers:
As WP8.0 apps are Silverlight, after retargetting it will still be a Silverlight app but WP8.1 (some new features and enhancemens).
Thought, if you want to retarget to WP8.1 Runtime, it's a totally different thing - it's a different type of an app, different API and more. You will have to port it manually. Note that not all features from WP8.0 are yet available in WP8.1 Runtime - here is a good article about that.