Linden Lab Official:What happens to land I own on the mainland if I downgrade from a Premium account to Basic?
Basic accounts can't own land on the mainland. If you're going to downgrade from your Premium account to Basic, you should relinquish your land holdings on the mainland by abandoning or selling your land. You are billed for your land holdings for the last billing cycle.
If you downgrade while the land is still yours, we will continue to charge you for it. Since it's highly unusual for Basic accounts to own land on the mainland, your account may be automatically held for review and your land may be reclaimed from you.