I am booked on 919/1565 LHR-IAD-MCO on 30th April. I booked through a travel agent.

I want to see if it's possible to move to 923/1561 instead as this arrives at MCO 3 1/2 hrs earlier. When I priced both on ua.co.uk the 923/1561 comes out cheaper, so in theory should my agent be able to book this, or can i ring reservations and see if they can move me onto 923/1561 ?