366. It is not permitted to substitute other chants for those found in the Order of Mass, for example, at the Agnus Dei (Lamb of God).
367. In choosing the chants between the readings, as well as the chants at the Entrance, at the Offertory, and at Communion, the norms laid down in their proper places are to be observed (cf. nos. 40-41, 47-48, 61-64, 74, 86-88).
As for 366, my sense is that American substitutions for the Order of Mass have never been widespread, with the possible exception of memorial acclamations. Much more of a problem in Europe, and even at the Vatican, according to what two touring choir directors told me in the 90′s.