May the priest or the deacon change the words at Mass?

The answer comes to us from Father Thomas Keller, who is archdiocesan master of ceremonies and associate director at the archdiocesan Office of Worship.

Sometimes. The liturgical texts direct us to true active participation where we offer ourselves with Christ on the altar of the cross in the sure and certain hope of the resurrection. Many of these prayers have nourished saints and sinners for nearly two millennia. Some have emerged from the collective wisdom of the Church in the modern world. All of them belong to the people in the pews.

All liturgical texts may be divided into two kinds. The first kind is where the Roman Missal (Sacramentary) offers the option "in these or in similar words." Here, the sacred minister may vary the texts to help direct the assembly to greater participation.

The other texts in the Missal may not be changed. The Second Vatican Council’s "Constitution on Sacred Liturgy" offers this general norm: "(N)o other person, even if he be a priest, may add, remove, or change anything in the liturgy on his own authority" (No. 22.3).

The regulation of the liturgy belongs solely to the Holy See and where the law determines, the local bishop.

Also, the new instruction on the liturgy, "Sacrament of Redemption," insists on adherence to the proper texts: "The reprobated practice by which priests, deacons or the faithful here and there alter or vary at will the texts of the Sacred Liturgy that they are charged to pronounce must cease. For in doing thus, they render the celebration of the Sacred Liturgy unstable, and not infrequently distort the authentic meaning of the Liturgy" (No. 59).

It is an abuse of power to change the words for novelty, to distort theology or even to increase one’s own popularity. In doing so, the assembly is robbed of its right to fully participate. The prayers are deprived of their power; instead the power resides in the one who alters them.

Now if only this were put into practice...

Source.