not change the changable ticket

Hello, I got a return ticket from budget air and I chose the changable ticket by paying extra money. Now I'm trtying to change my ticket but they say that airline doesn't allow to change your ticket. So, if the airline doesn't let the ticket change, why you sold me changeble ticket and deveive me. I contacted with customer service and their only solution for me to buy a new ticket..what a solution huh? I'm totally dissatisfied with the service and do not recommend budgetair to anyone.