You probably don't have much of a choice. When you change dates it's like making a brand new reservation. Your reservation is re-priced at the prevailing rate for your new dates. It makes absolutely no different what your price was before for your old dates. Unfortunately, being Preferred Plus doesn't help you here.
Your only hope is that if you booked a deal that can be cancelled without penalty (or a small one). Then you can cancel and shop around for better deals.