Changing Flights

Plan B. After not being able to get a refund, (refer to an earlier post) I thought I'd just change the dates on my tickets. Well I checked yesterday to change one of the flights and it was was $233 which was OK (just OK). Checked with friends and decided to book the flight today ---SURPRISE $483 to change, what can change so much in 24 hours ? Frustrated but will still not give up!!!! And Melbourne was the only city that didn't have flights to Honolulu on special...huh????? Jetstar do make it difficult.

International
Australia-Melbourne - Tullamarine
United States-Honolulu