Dropping the last leg is ok but not the first.
When you change your flights the airline is giving you credit from the old ticket price - a change fee and applying it to the new ticket. You would be free to change both legs if you wanted but the refare may not save you any money. You may be best just purchasing a new ticket. It depends on the airline and the change fee. If you were flying united and the original flight cost under $150(if there is a flight under $150) you would receive no credit because the cost of the flight would be less the the change fee. If the flight cost $300 and the new flight costs $450 you total out pocket expense is is $600. $300 for the orignal ticket-$150 change fee=$150 credit. The new flight costs $450-$150 credit= $300. Total cost $300 orignal ticket + $300 new refare ticket cost= $600.