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, 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. 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. In this, it differs considerably from the decree Divinam Psalmodiam of Urban VIII (1631; it imposed the text of the hymns confected by the Pontiff and his fellow admirers of Horace in place of texts in the Latinity which had been good enough for the likes of Ambrose and Venantius Fortunatus). Urban's decree is full of fire-breathing menaces for anybody who shall print unamended texts after the decree, but allows 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.
S Pius V, as we all know, did not impose his revised texts on any Church which had its own local 'dialect' (the term is Fortescue's) of the Roman Rite which was more than 200 years old. But he did take a rather fierce line with those not in that position. In effect, the Bishop of Rome was saying, not unreasonably, that if you use the Roman Dialect of the Roman Rite, you shall use it in the form in which I have revised it. This, of course, was in the period of retrenchment when the Latin Church was on a war-footing against Protestant enemies who delighted to find corrupt or indefensible texts in popish service books. In the circumstances, it is rather remarkable that the Pope was prepared to tolerate local 'dialects' at all. It probably demonstrates that, despite all his centralising bubbles and froth, S Pius was not certain that he could afford to get embroiled with powerful local primacies which had their own entrenched usages.
It is my view that a rough but good rule of thumb as to whether a 'reform' is or is not 'organic' is the consideration: does it render all existing liturgical books totally obsolete after a certain date? When people defend the process of imposition under Paul VI of his new books by reminding us that changes not inconsiderable had been made before (as some fool in the Tablet did a couple of months ago), I don't think they realise the depth and rapidity of the Pauline rupture, compared with (undeniably real) earlier discontinuities.
And yet, curiously, although the Pauline decree Laudis Canticum was so explicit in displacing and suppressing the Breviary hitherto in use, 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 so. But I presume that their funny little lapse was the ground upon which that 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.