What is the time between your outbound and your return?
Doing changes before the outbound flight means that fares available when you do the change are used, and that may be very high fares. Changing after the outbound is flown may allow to use fares and advance purchase requirements based on the time of original ticketing. So sorting it out right now may not be the best solution. However figuring this out does require some expertise... And if there is little time between the outbound and return flight, the low fare booking classes may not be available. Can you tell us the origin and dates of the original fare as well as the booking class? With that info, someone on here could lookup historical fare rules. That would be a good starting point.