First of all, they’re not the same, so if you’ve already created Windows 8 app you will need to consider upgrading them.
On a windows 8.1 device, you will be able to use both the old windows 8 apps, as well as the new windows 8.1 apps. On a windows 8 device however, you will only be able to use windows 8 apps; in the app store, windows 8 users will only see windows 8 apps, so 8.1 apps will not even be visible. For windows 8.1 users, the 8.1 apps will appear in search results first, so it is recommended to upgrade your apps if you want to keep your ranking up.
Windows 8.1 apps can only be created and edited in Visual Studio 2013, and upon opening an older windows 8 app in Visual Studio 2013 you will be prompted to upgrade the solution. After following the instructions to upgrade the solution, your windows 8 project will be upgraded to 8.1.
Without even making any code changes, your app will instantly be faster than before in both the initial start-up and general app use.
Visual Studio 2013 can be used to edit and maintain older windows 8 apps, however to create new windows 8 compatible apps, you need to use Visual Studio 2012, which can run alongside Visual Studio 2013.
Some code has been deprecated with 8.1 apps, for example all the state-related code is now redundant as Windows 8.1 does away with the ‘snapped’ views in exchange for freely-sized apps from full screen right down to 500px or 320px. App developers will need to update any code that checks for the ‘snapped’ state, and replace it with direct checks to the apps ‘width’ property.
When opening an app in Visual Studio 2013, you will be prompted if your solution has any deprecated code so that you know what needs to be updated.
Apps will now optionally be able to set a minimum width of either 500px, or 320px (where 320px was the ‘snapped’ equivalent in windows 8).
When uploading your app to the Windows App Store, you will be able to upload both a Windows 8 version and a Windows 8.1 version within the same store submission, so that it will share the same Name and Publisher etc. You will however need to specify a unique Description, Features, and Screenshots for each, as you are likely to be changing some things or adding new features with the 8.1 updates.
One important thing to note when creating your 8.1 app side-by-side to your 8 app is that the 8.1 app version number CAN NEVER exceed the version number of the 8.1 app where you FIRST uploaded it. For this reason it’s a good idea to give the windows 8 app version number plenty of room for increments before the windows 8.1 app version number starts.