Zero Downtime App Upgrade on GAE

Here's the scenario for such a trick:

1. Prepare new version of your app, and deploy it as some other version that current production has
2. Go to Versions tab of your app's dashboard at http://appengine.google.com and get URL of the newly deployed version
3. Check your new version of app using this URL
4. On the Versions tab, select new version and make it default.

That's it!