When exchanging your tickets you may find prices vary by performance. Exchanges can only be made into seats at the same price as you originally paid or higher.
Many productions vary pricing depending on the date and time of the performance, and may offer peak and off-peak prices. This is based on the popularity of a performance and the ticket availability.
Tickets can only be offered at the price they are currently advertised at. Prices can change at any point during the show's on-sale period.