1. Changing date before departure will cause the fare to re-price. So you're looking at easily a few more thousand dollars per passenger.
2. Changing the date en-route. You will need the ticket issuing carrier involved here, and they may get nasty especially if the last 2 flights do not involve the issuing carrier.
My suggestion is not to be greedy and work with the dates already booked. Or just refund and call it a day. Otherwise you should be mentally prepared to have your itinerary chopped. If you don't have a back up plan to get yourself back home if the airline chops your last 2 sectors, then don't do it.