RobertH is correct. If you change the outbound of a ticket you have already purchased, you are essentially purchasing a new ticket right there. The airline then allows you to use the value of your previous ticket plus the change fee of the original ticket.
If you are only changing the return, only the change fee will apply if the change you are making falls within the rules of the ticket. I.E. Saturday night stay, Tues Wed only fare, etc. Unless again, the original class of service is not available, then some fares will allow you to upgrade the return to a class of service that is available, but not all fares allow this.

------------------
"Work like you don't need the money, and dance like you do when nobody's watching."