Hi @Freakyrockstone
The content you've collected, like personal playlists, albums and songs will continue to be saved to your Pandora account for the next time you upgrade to Premium or if you use a Premium Access session.
Your Offline content will be removed if you downgrade to the ad-support version. However, if you subscribe to a Plus subscription, the three most listened to stations and your Thumbprint Radio will be available for offline listening.
Hope this helped to answer your question.
Freakyrockstone