When you downgrade your campaign tier, you will keep your current tier until the end of the billing cycle. Then, you will lose access to any features not in your tier. However, your data will remain there waiting for you. If you upgrade your campaign again, you will regain access to those features immediately.
If you have more players than allowed in your tier, you will be asked which players you wish to keep at your table. The rest of the players will be locked out, but will remain until removed.
If your File Hub storage is too full, those files will remain there, but you will not be able to add anything new until you are within your tier limits again. You will also not be able to view the file storage list.
If you have selected Image Library images, the images will remain selected even if downgraded, however you will not have access to the library again until on a supported tier.