When you downgrade from a paid account type to a free one, you have less storage available in your Library. If you exceed your new storage limit, lessons will be archived. They won't be deleted permanently, but you won't have access to them.
Once you re-upgrade, these lessons will be restored automatically, up to your new storage limit. If for some reason you seem to be missing lessons you can reach out to email@example.com and we will be happy to assist you with recovery.