Hi @carl00gratton. Upon canceling your premium subscription you will maintain any offline tracks you have until your premium ends (e.g. if you cancel on the 4th, but you have premium until the 10th, you will still be a premium user until the 10th). Once you revert to a free account, any offline tracks you have will be removed from your devices and you will not be able to listen to them offline. If you decide to return to premium you will have to re-download these songs again.
Hope this clears things up for you. Please let me know if you have any other questions or concerns that I might be able to assist you with. Cheers!