How to access both internal AND external persistentdatapath on Android?

I've always had "preferexternal" in my AndroidManifest.xml, however I didn't have the permissions to write to external (so it was always saving to internal memory). I recently updated my app with a new plugin that added this permission. Now anyone who has an sd card (external storage) is updating to the new version and it gets installed to external storage... but their old save data is still in internal storage! Of course people are upset that they lost all data! If I remove the permission and post a new update, anyone who has played since the first update will lose THEIR data. People who played with both updates will lose progress TWICE. This is a disaster! What I need is a way to access both the internal AND external save locations (if both exist) and merge the save data found in each location. How can I explicitly choose one or the other when I have permissions for both? PLEASE HELP!