When you downgrade from Pandora Premium to Pandora Plus or ad-supported Pandora, you will not be able to access your on-demand music collection or custom playlists. However, your collection and playback history will be stored in our servers for when you return to Pandora Premium. You may need to re-download those songs upon resubscribing.
In the meantime, you will have access to all the stations you've created and any podcasts you've added to your account including all Thumb feedback and added variety.
Alternatively, you can also unlock complimentary sessions of Pandora Premium by searching for a song or album and pressing Play. Then your full collection, including any playlists, will be visible again.