Looking for some guidance on changing an award.

By a miracle, I was able to score 4 award tickets to Hawaii over X-Mas, MSP-KOA via SLC on Delta. Now I am interested in changing the return date. By another miracle, per the flight search tools, there is availability on my new desired return date, also on DL. However, when I attempt to change the reservation on nwa.com, I can only see the NW flights, all of which are rulebusters and require more miles.

First question: For a change to reservation, is there a way I can see the DL flights?
Second question: Am I correct that changes to awards are free of charge on nwa.com but require a fee if I use an agent? (I'm cheap).

Thanks!
TP