Some years ago, while looking through the library of the late and learned and very lamented Fr Michael Melrose, Successor Martyris as Vicar of S Giles, Reading, I spotted an unusual little volume (well, there were plenty of those: what a Library!): very slender, published in 1912, it gave the Psalter as rearranged by S Pius X. In other words, when S Pius made his revolutionary changes to the distribution of the psalms, you didn't have to buy a new Breviary; you bought the Slender Volume and used it in conjunction with your old Breviary.
But you did have to make some such provision to say the psalms in the new arrangement. The Decree Divino afflatu makes clear that if, after a certain date, you fail to fall in with the new order of things, you are not fulfilling your obligation to say the Divine Office. Fierce!!
In this, it differs considerably from the decree Divinam Psalmodiam of Urban VIII (1631). Urban's decree is full of fire-breathing menaces for anybody who shall print unamended texts after the decree, but he permits books already printed to go to the booksellers ... and books in the bookshops to be sold ... and books in use to continue to be used. In other words, Urban was content to rely on a gradual process of books wearing out and being replaced.
Something like this human and common-sense approach can be found as late as 1902 in the Edition of the Ambrosian Missal promulgated that year by Andrew Cardinal Ferrari. He required his new edition to be used "in virtute sanctae obedientiae", but with this let-out clause: "Concedimus tamen, aequis de causis, ut donec a Nobis aliter disponatur, vetera approbata exemplaria adhuc adhiberi possint; ita tamen ut nullum eorum ex quocunque titulo abhinc acquiratur ad Sacram Liturgiam peragandam". In other words, if you've got a previous edition in your sacristy, you may continue to use it, but you mustn't "acquire" another copy of that old edition.
It is my view that a rough but good and healthy rule of thumb as to whether a 'reform' is or is not 'organic' [vide Sacrosanctum Concilium of Vatican II] is the consideration: Does it render all existing liturgical books totally obsolete after a certain date? When people defend the process of imposition under Blessed Paul VI of his new books by reminding us that changes had been made in earlier times, I don't think they realise the depth and rapidity of the Pauline rupture, compared with pre-1950 discontinuities. The same is true, of course, of the slash-and-burn approach adopted by Pius XII and his side-kick Bugnini to the ancient Roman rites of Holy Week.
Nobody is entitled to disagree with me about this if they have not compared, firstly, the Missal of S Pius V with the first printed edition of the Roman Missal a century earlier; and, secondly, the Missal of B Paul VI with that promulgated by his predecessor less than a decade earlier.
And there is no way that the printers could have confected a Slender Volume aided by which you could use an old Missal to say the Novus Ordo. The changes are vastly too massive.
And yet, curiously, although B Paul VI's decree Laudis Canticum was explicit in displacing and suppressing the Breviary hitherto in use, his decree Missale Romanum did not state that the Old Mass would be illegal after the New came into use. Was that an oversight? Did the canonists drafting it think that it was too obvious to need saying? I suspect that something like this may be the answer.
My theory is that this funny little lapse was the ground upon which a Commission of Cardinal canonists decided by a majority vote that the Old Missal was not abrogated - a verdict finally published and confirmed in the motu proprio Summorum Pontificum.