The 1975 General Instruction of the Roman Missal (GIRM) had:
“11. It is also up to the priest in the exercise of his office of presiding over the assembly to pronounce the instructions and words of introduction and conclusion that are provided in the rites themselves. By their very nature these introductions do not need to be expressed verbatim in the form in which they are given in the Missal; at least in certain cases it will be advisable to adapt them somewhat to the concrete situation of the community. (Footnote: See SCDW, Circular letter on the Eucharistic prayers, 27 April 1973, no. 14 [DOL 248 no. 1988].)”
(From Documents on the Liturgy 1963-1979, Liturgical Press, Minnesota, 1982, ISBN 0-8146-1281-4).
What exactly were “words of introduction” was probably debatable, but I think they would have reasonably applied to the first three examples Facite gave.
The current 2002 GIRM has replaced this with:
“31. It is also up to the priest in the exercise of his office of presiding over the assembly to pronounce the instructions that are provided in the rites themselves. Where it is indicated in the rubrics, the celebrant is permitted to adapt to some extent these remarks …”
The rubrics do not permit variations with the examples that Facite gave, so varying them is no longer permitted.
Some of the things the GIRM permits:
“23. Moreover, in order that such a celebration may correspond more fully to the prescriptions and spirit of the Sacred Liturgy, and also in order to increase its pastoral effectiveness, certain accommodations and adaptations are specified in this General Instruction and in the Order of Mass.
24. These adaptations consist for the most part in the choice of certain rites or texts, that is, of the chants, readings, prayers, explanations, and gestures that may respond better to the needs, preparation, and culture of the participants and that are entrusted to the priest celebrant. Nevertheless, the priest must remember that he is the servant of the Sacred Liturgy and that he himself is not permitted, on his own initiative, to add, to remove, or to change anything in the celebration of Mass.”
“50. … After the greeting of the people, the priest, the deacon, or a lay minister may very briefly introduce the faithful to the Mass of the day.”
“128. After the Collect, all sit. The priest may, very briefly, introduce the faithful to the Liturgy of the Word.”