Converting an existing user to a restricted profile With Android 4.3 comes a new feature, restricted profiles. It allows creating new special "restricted" users, for which an admin user can control access to apps.

However, in addition to my own admin user, I already have a secondary user for my kid, which has all of the user-specific data, such as saved games. If I create a new restricted profile, those will be lost.

How can I convert an existing (secondary) user to a restricted profile?

Note for the interested: The restricted profiles feature doesn't provide a complete secure access block, but it works well as a hinderance and prevents accidental blunders - which happens to be excatly what I'm interested in.
As of this moment, it is not possible to convert a user into a profile. 

However, you could create a profile based on the existing user. Downside: Since a profile doesn't have a Google account tied to it, they can't have a GMail app. And as you have already mentioned, they would also lose those user-specific data.

UPDATE #1: From Android.com, it says that a restricted profile is a subset of a user's account. It's using the same user account, albeit restricted. So the profile is quite different from a User, where it's tied to a different Google account.


  Each restricted profile offers an isolated and secure space with its
  own local storage, home screens, widgets, and settings. Unlike with
  users, profiles are created from the tablet owner’s environment, based
  on the owner’s installed apps and system accounts. The owner controls
  which installed apps are enabled in the new profile, and access to the
  owner’s accounts is disabled by default.