I am making an android game with both a demo version and a paid version. I want users who decide to buy the paid version to continue from where they left off, with all their old data and settings.
I've set a shared user id between both applications, and now I'm wondering if it's possible to use libgdx preferences across both versions.
I am also open to other ideas on how to implement this.
You will not be able to do this with the libgdx API because this is more closely related to how android handles preferences.
Internally, libgdx uses android.content.SharedPreferences to store the preference data. You will have to write android code to make the preferences globally readable and then you will be able to access it across applications. An example of how to do this can be found on a stackoverflow question here.
