From a reader…
Can a priest change the words”for many “ to “for all?” The priest said it’s optional.
No. A priest may not by his own authority change the liturgical texts. That would be a liturgical abuse. The priest does NOT have the option to do that.
Furthermore, “for all” is precisely what the Latin “pro multis” does not mean. “For all” is precisely what the Church has officially taught for centuries that should not be said.
And this liturgical abuse is graver than others by the fact that it concerns the form of the sacrament, the words of consecration of the Precious Blood.
Since it seems that you have already spoken to the priest about it, you should get a recording of the priest saying this and send it to the local bishop. If that does not produce results, send it to the Congregation for Divine Worship in Rome.
What that priest is doing does not invalidate the consecration, but it is still a big deal.
The Church, much less an individual priest or bishop or Pope, cannot blithely change the language of the text, which has an official Latin foundation, from pro multis to pro universis (for all). That would explicitly contradict the Church’s teaching as expressed in Latin by the Council of Trent (cf. Catechism of the Council of Trent, Part II, 4). Such a change would contradict doctrine and not simply change emphasis about an aspect of that doctrine. The English must reflect the Latin.
UPDATE 6 Feb 2020:
A good point has been made in the comments.
It could be that the priest just slipped back into an older mode.
I remember an old priest who, late in the pontificate of John Paul II, slipped into “for Paul our Pope”.
I myself have to concentrate hard on the rare occasions when I say the Novus Ordo, not to do certain things, which are now powerfully wired in as muscle memory. And even though I have written extensively on the propers of the Novus Ordo, the new translation of the ordinary is still “new” to me.
So, it is possible that the priest in question just slipped.
However, the questioner said she asked the priest and he responded that he had the option.
No. He really doesn’t. That suggests that he knows what he is doing.