Can you provide chapter and verse of the document(s) that forbid this practice?
On the matter of changing the words of fixed prayers during the Mass, the Catholic Church teaches:
Dominae Cenae, element 12
“The priest… cannot consider himself a “proprietor” who can make free use of the liturgical text and of the sacred rite as if it were his own property, in such a way as to stamp it with his own arbitrary personal style. At times this latter might seem effective, and it may better correspond to subjective piety; nevertheless, it is always a betrayal of that union which should find its proper expression in the sacrament of unity.”
“Every priest who offers the Holy Sacrifice should recall that it is not only he with his community that is praying but the whole Church, which is thus expressing in this sacrament this spiritual unity, among other ways by the use of the approved liturgical text. To call this position “mere insistence on uniformity” would only show ignorance of the objective requirements of authentic unity, and would be a symptom of harmful individualism.”
Code of Canon Law, 1983, Paragraph # 838
"The supervision of the sacred Liturgy depends solely on the authority of the Church which resides in the Apostolic See and, in accord with the law, the diocesan bishop.
“It is for the Apostolic See to order the sacred liturgy of the universal Church, to publish liturgical books, to review their translations into the vernacular languages, and to see that liturgical ordinances are faithfully observed everywhere”
“It pertains to the [national] conferences of bishops to prepare translations of the liturgical books into the vernacular languages, with the appropriate adaptations within the limits defined in the liturgical books themselves, and to publish them with the prior review by the Holy See.”
“It pertains to the diocesan bishop in the church entrusted to him, within the limits of his competence, to issue liturgical norms by which all are bound.”
Code of Canon Law, 1983, Paragraph # 846.1
“The liturgical books approved by the competent authority are to be faithfully observed in the celebration of the sacraments; therefore, no one on personal authority may add, remove, or change anything in them.”