There is no guarantee of infallibility for Church policy. A moment's reflection bears this out, for if they were matters of faith, then they would have to be, like the Faith itself, the same in all times. But this has never been the case. So Quo Primum itself is based on the obvious principle that the liturgy is capable of change. The confusion is on two points.
Liturgical directives are disciplinary and, therefore, subject to change.
No theologian in the entire history of the Church, I think, has ever maintained the contrary position, and certainly the Church has never taught the contrary position which is, in any case, absurd on its face. The second is the confusion about the binding force of Quo Primum . First, the Pope again shows that he understands that there is nothing necessary to salvation or irreformable about the particular form which his new Missal takes. However, if this Missal, which we have seen fit to publish, be more agreeable to these latter, We grant them permission to celebrate Mass according to its rite, provided they have the consent of their bishop or prelate or of their whole Chapter, everything else to the contrary notwithstanding. Clearly, if the Pope believed that celebration of the Mass precisely according to the new missal was necessary to salvation or a matter of Faith, then he could not have admitted any exceptions whatsoever.