Where the contract of carriage is modified so that:

1. (a)

   a transport operation which was to end outside the customs territory of the Union ends within it, or

2. (b)

   a transport operation which was to end within the customs territory of the Union ends outside it,

the authorised railway undertakings shall not perform the modified contract without the prior agreement of the customs office of departure.

In all other cases, the authorised railway undertakings may perform the modified contract; it shall inform the customs office of departure of the modification made without delay.