I assume that the IAD-LAX flight you're talking about is UA 201 9:10am - 11:32 am? (The only IAD-LAX I show as being a 762)

I assume since you say your Hawaii flight is on a 757, you're currently booked either on LAX-OGG UA 39 4:45pm - 7:13pm or LAX-LIH UA 31 4:05pm - 6:52pm? (I show all LAX-HNL on either 763 or 777 aircraft.)

Or do I have your LAX-Hawaii connection wrong?

If my guessword is correct, you have plenty of time to take a later IAD-LAX flight.

Either:
UA 187 IAD-LAX 1100-109pm on an A320
UA 39 IAD-LAX 1255-303pm on a 757 (in fact, this flight continues to OGG)

Do either have confirmable upgrade space available? If so, you might switch.

That said, I've had aircraft switched on me and wound up in 3-class 1st... and if the change were to be made at the airport rather than in advance, the fact that you're on your honeymoon may gain lots of sympathy.