Changing from free app to paid app on Google Play

You could add in-app payment processing to your free app and charge for enhanced features. Per

http://support.google.com/googleplay/android-developer/bin/answer.py?hl=en&answer=1153479

"By using the in-app billing feature you can add revenue-generating features to your applications, enhancing the revenue potential of paid applications and turning free applications into new revenue sources."

Now, could you disable functionality that was already available in the free app, and then charge via in-app payments to have it restored? Technically I do think it could be done, since you are in control of your implementation of those features, although you would have to investigate whether that would be a violation of Google's policies.

However, doing it might alienate your users, and if word got around they might even stop updating your app to avoid the "trap." So a much better approach would probably be to add enticing new functionality that enhances the existing functionality, and then charge for it.

You cannot. Check this

"Once your app has been offered for Free, the app can't be changed to Paid. If you want to charge for the app, you need to create a new app with a new package name and set a price."

You cannot change a free app to a paid app on the Google Play store. You could re-upload under a different name, but this would obviously fracture your user base.