You can't downgrade a business card to a personal card, only to another business card, and you may not be able to downgrade a United card to a non-United card. That leaves extremely few options.

But why do you need to downgrade a business card? Why not just cancel? Chase business cards doesn't show on credit reports, so the cancellation of it won't show up on credit reports either. So who besides you will know the difference between cancelling it and converting it (if it were possible)?