If a German unit moves into a hex occupied by an Allied Truck, does the Truck get a chance to move away to avoid being commandeered, since it is not actually commandeered until the Board Upkeep phase?
No. Trucks are captured as soon as enemy combat units move into their hex, even though they are not commandeered until the Board Upkeep phase. When they are captured, they may no longer be used by their former owner. However, they also may not be used by their new owner until they are commandeered. This interrim period (the remainder of the Movement and Reinforcement phase) represents the time necessary to take prisoners and assign personnel to the newly captured Trucks.