the only "money" needed if you did that....would be if the flights you are now booking cost more than your original ones,
otherwise they do not charge change "fees" per se
you might have to cancel what you have to get "ticketless" funds and then use those funds to purchase the new tickets
or maybe you might want to call reservations to see what they can do
the only bonus to doing it on the net, if you have a rapid rewards number you get extra .5 credit for each leg of your flight if you book it ONLINE
hope this makes sense