At least for many fares it seems that changes after the outbound has been completed should be recalculated using historical prices. I guess that in this case, if the same fare-bucket is available there would only be a change fee, but if for example it was an I fare and there was only higher fare-bucket availability for the new date there would also be a re-pricing (in which case it may or may not be cheaper than a repricing into a lower fare-bucket at current prices). The OP may want to take a look at flightstats and then ask him/herself "do I feel lucky" ...