It is something that Phone Team has been attempting with some success and they now run the entire OS Team. So far it has worked but like I said they ran into a Showstopper with the Windows Phone 8.1 Update 1 roll out with encrypted phones, but they fixed it eventually (4 months later).I assume you mean an in-place upgrade. I don't ever remember seeing that before, only a final version to a newer version. Interesting!