This is very much to the purpose of the thread. When did this change happen? Are there Church documents, writings of saints or any other relevant writings on the Catholic side denying that the Greeks had valid Sacraments?
This is a well known debate within the Roman Communion under the Pope, and I am sure an archive search in the Traditionalist section of CAF would pour out several threads on the subject.
I would have to submit the following as a primary source. Some have claimed this Bull to be infallible, but others dispute this. I think this would have to be considered part of the Ordinary Magisterium of the church in it’s time though, as far as I understand that.
"The most Holy Roman Catholic Church firmly believes, professes, and preaches that none of those existing outside the Catholic Church, not only pagans, but also Jews and heretics and schismatics, can have a share in life eternal; but that they will go into the eternal fire which was prepared for the devil and his angels, unless before death they are joined with Her; and that so important is the unity of this ecclesiastical body that only those remaining within this unity can profit by the sacraments of the Church unto salvation, and they alone can receive an eternal recompense for their fasts, their almsgivings, their other works of Christian piety and the duties of a Christian soldier.
No one, let his almsgiving be as great as it may, no one, even if he pour out his blood for the Name of Christ, can be saved, unless he remain within the bosom and unity of the Catholic Church."
Bull Cantate Domino, Council of Florence by His Holiness Pope Eugene IV 1442AD
This follows the Bull Unam Sanctam of about 140 years earlier …
“Declaratio quod subesse Romano Pontifici est omni humanae creaturae de necessitate salutis”
" we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff."
The span between the first quote and the second could represent six to seven generations of teaching on the subject, and actually 14 Popes from the one to the other.
Now we are aware that many fathers from the first millenium church did see rival churches teaching seriously heretical doctrines as graceless. This would be a continuation of a very old theological opinion. The point here is that those in schism quite apart from heretics are also unable to be saved.
Granted, this does not address the efficacy of Orthodox sacrament directly. What it says is that anything and everything one does outside of the Roman Catholic church avails one not a bit. I think one could reasonably presume that also means receiving a sacrament from a priest not under the Pope (in schism) is pointless.