Thanks for searching for the answer you need here!
No worries, changing the subscription type won't affect the music and contents saved to your Spotify account.
In case your account gets on free between the two subscriptions only your downloaded content will be removed from the device, but you can easily retrieve it after getting back on Premium. Here you can find more info on how to do that.
Hope this helps. Let us know if you have any more questions.
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.
“Music is the strongest form of magic.” Marilyn Manson