Is it possible to update an existing Windows Phone 8 app to Windows Phone Store 8.1

Question

I\'ve a Windows Phone 8.0 app on the Windows Phone Store, and I want to update my app to Windows Phone store API (and not Windows Phone Silverlight 8.1) to prepare Windows 8

Answer 1

TL;DR; - It preserves data on Isolated Storage when updating from WP8.0 to WP8.1 Runtime.

Because the provided link to MSDN says only about Silverlight apps, and it's not clear (if I hadn't missed something) what would happen in case: I've old WP8.0 Silverlight App and now I decided to upgrade it to WP8.1 Runtime - I've performed a simple test:

1. I've published a Beta version of the App - WP8.0 Silverlight.
2. After successful instalation on the Phone, I've copied some files inside the App, so that there is something on Isolated Storage
3. I've submitted an update - went to Store, selected Beta version (WP8.0), clicked Update then Upload and describe your packages, after a while, Add new (not replace the old WP8.0 package), add some information, save and submit.
4. After some time my Phone is notified that there is an update for the App - I click yeah - update
5. After successfull instalation, I see that it's a new App, I click my special button to see what is on Isolated Storage and I see that there are old files from WP8.0 version

So it turns out that LPains answer is correct (+1).