All types of changes are subject to the airline's availability and the rules of your specific ticket. 

If you have a MultiFLEX pass you may be able to change your flights without change fees.  The MultiFLEX pass covers the change fees but any additional fare and taxes are still payable when you change. If you do not have a MultiFLEX pass you will need to pay the airline and STA Travel change fees plus any applicable fare/tax difference.