It has been an while since I switched around flights but NW has always been very flexible, even when I was silver. I don't think you need to get to the airport way early, just make sure you have your boarding pass and are ready at the gate a little more than 1 hour before departure of the LAX flight. You can make it a win-win by making your request because you "are worried the flight from MSP to LAS is overbooked" (it looks pretty full). That may give them an extra incentive to consider your request. If they start talking about switching connecting cities you can bring up Fly-Now and the $25.

It really depends very much on the GA, some go by the book while others are really service oriented. It may be possible to figure out who to ask by starting with an unrelated question, maybe about how full the LAX flight is.

Good luck!