Although I am not a liturgical expert, I do have some knowledge of the liturgical laws. I believe I can help you on this question:
At our parish (and many others), phrases like “Bread of Life” “Prince of Peace”, etc. are inserted into the Lamb of God when sung by the choir. “Lamb of God, …, have mercy on us” is actually only sung once, then the fillers are inserted, and the final line " … Grant us Peace" is then sung.
“Bread of Life” is listed as an “alternative” in our hymnal but is this licit? I don’t like it.
According to the General Instruction of the Roman Missal, this is not acceptable. There is no provision in it or the lectionary for substituting new tropes (“Bread of Life”, “Prince of Peace”, “King of Kings”, etc …) during the Agnus Dei. I cite:
“It is not permitted to substitute other chants for those found in the Order of Mass, such as at the Agnus Dei.”
G.I.R.M. Paragraph 366
Since this chant occurs during the fraction (the breaking of bread), and since the fraction may take a little more time, it is permitted to say/sing/chant “Lamb of god, who takes away the sins of the world, have mercy on us” more than the customary twice (the final time, as you know, ends with “grant us peace”). I cite:
The supplication Agnus Dei, is, as a rule, sung by the choir or cantor with the congregation responding; or it is, at least, recited aloud. This invocation accompanies the fraction and, for this reason, may be repeated as many times as necessary until the rite has reached its conclusion, the last time ending with the words dona nobis pacem (grant us peace).
G.I.R.M. Paragraph 83
Now, this came as a surprise to me. I had long been under the impression that the Agnus Dei must start and end with “Lamb of God” but could have other appropriate tropes in between.
A possible reason for this confusion is a publication called Liturgical Music Today, a 1982 set of guidelines compiled by the U.S. Bishops’ Committee on the Liturgy:
“The Lamb of God achieves greater significance at Masses when a larger sized eucharistic bread is broken for distribution and, when communion is given under both kinds, chalices must be filled. The litany is prolonged to accompany this action of breaking and pouring. In this case one should not hesitate to add tropes to the litany so that the prayerfulness of the rite may be enriched.”
Liturgical Music Today, Paragraph 20
There is, however, no evidence that the U.S. Bishops obtained appropriate approval from Rome for this deviation. Since the Constitution on the Sacred Liturgy, Sacrosanctum Concilium, requires in section 22 that they get such approval, one can only conclude that the practice of adding tropes to the Agnus Dei is improper. I cite:
- Regulation of the sacred liturgy depends solely on the authority of the Church, that is, on the Apostolic See and, as laws may determine, on the bishop.
- In virtue of power conceded by the law, the regulation of the liturgy within certain defined limits belongs also to various kinds of competent territorial bodies of bishops legitimately established.
- Therefore no other person, even if he be a priest, may add, remove, or change anything in the liturgy on his own authority.
Sacrosanctum Concilium, Paragraph 22
And so, in response to your question, no, it is not permitted to change the wording of the Agnus Dei (or any other prescribed text, for that matter).
I hope I have helped you out.
MT