If you have used more storage space than the available space after the downgrade, you will receive an e-mail notice, and your account will be unable to add new files or share files.
You can go to [web version] to check your space details.
You can save your files in the following ways.
1. Upgrade storage space
You can go to [plan page] to choose a plan which is larger than the current used space to avoid file deletion.
2. Free up storage space
You can reduce the files you stored in the cloud to less than the total space to avoid file deletion.