It is not permitted to substitute other chants for those found in the Order of Mass, such as at the Agnus Dei.
The norms laid down in their proper places are to be observed for the choice of the chants between the readings, as well as of the chants at the entrance, at the offertory, and at Communion (cf. nos. 40-41, 47-48, 61-64, 74, 86-88).