Who Says Ya Can't Go Back?

From Quo Primum:

Furthermore, by these presents [this law], in virtue of Our Apostolic authority, We grant and concede in perpetuity that, for the chanting or reading of the Mass in any church whatsoever, this Missal is hereafter to be followed absolutely, without any scruple of conscience or fear of incurring any penalty, judgment, or censure, and may freely and lawfully be used. Nor are superiors, administrators, canons, chaplains, and other secular priests, or religious, of whatever title designated, obliged to celebrate the Mass otherwise than as enjoined by Us. We likewise declare and ordain that no one whosoever is forced or coerced to alter this Missal, and that this present document cannot be revoked or modified, but remain always valid and retain its full force notwithstanding the previous constitutions and decrees of the Holy See, as well as any general or special constitutions or edicts of provincial or synodal councils, and notwithstanding the practice and custom of the aforesaid churches, established by long and immemorial prescription - except, however, if more than two hundred years' standing.

snip-

Therefore, no one whosoever is permitted to alter this notice of Our permission, statute, ordinance, command, precept, grant, indult, declaration, will, decree, and prohibition. Should know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul.

Given at St. Peter's in the year of the Lord's Incarnation, 1570, on the 14th of July of the Fifth year of Our Pontificate.

I especially like that big about incurring the wrath of Almighty God. We could do with more directives from Rome that include that.