I believe you can change to any destination in the same zone (ie that would have required the same number of Avios to book). As DXB is a 20,000 Avios (Zone 5) destination, I think that would limit you to one of the short east coast destinations - but NYC would work for that.
The exact words of the relevant bit of T&C are:-
16.3. The route applicable to a Reward travel booking may be changed provided that the new route selected would, as at the date the change is requested, require the redemption of the same amount of Avios points as was redeemed in making the original booking. Route changes are only permitted for Reward travel bookings which (other than for the payment of taxes, fees and charges) are purchased using Avios points only.