Data disappears when downgrading

Hi. I have two listing packages for users to choose from, (1) a free Basic package that offers a few fields to fill out, and (2) a paid Premium package that offers several more fields to fill out. I have noticed that when I downgrade a listing from Premium to Basic, the data that belongs to the Premium-only fields, gets deleted. When I then upgrade the same listing back to Premium, the former data for the Premium-only fields are gone. I would like the data to be retained upon downgrade, but just not displayed. The idea is for that data to reappear when the listing is upgraded back to a Premium listing. How can I solve this problem?