What constitutes a tradition? When can you say a tradition has changed to become a new tradition, and how long does that take? These are all tricky questions! However lets back up for a second and define our terms.

What is traditional Greek religion?

Popular understanding of Greek paganism is not all that accurate to the day to day reality of the past. When we think of Greek gods and goddesses we think of the 12 Olympians with their clearly defined roles and demesnes and a smattering of other deities. Zeus has the lightning bolt, Hades rules the underworld, Poseidon likes horses and the ocean, Hera gets cheated on, Aphrodite causes people to cheat, etc... This obscures more than it elucidates however! "Traditional" Greek paganism was wildly different in different corners of the Greek world. Certain gods, goddesses, and aspects of them were in favor or not depending on local preferences. Others were not worshiped at all. Many places traced their ancestry and founding to specific deities, demigods, heroes, etc... And the version that has come down to be taught in middle school mythology classes and filling children's books of mythological stories is ultimately only a tiny sliver of the existing religious traditions of Greek speakers.

However, this is not the religious tradition that Christianity stamped out. There were not throw downs in the agora of Greece over who was cooler, the Virgin Mary or Athena. "Traditional" Greek religion was under attack long before Paul started sending letters to the Corinthians et alea. In the long march of history from the time of Classical Greece to the Hellenistic period to the Roman period, religious traditions changed, slowly at times, but inexorably. New deities came into focus, others were subsumed or merged, new cults rose and fell all the time. At the time of the advent of Christianity within the Empire there were a wide variety of different faith systems, and trying to parse them out individually isn't always easy. Many of these were "Greek" or at the very least Hellenistic, but they were not exclusive to the Greeks, and they often bore little resemblance to the Olympian centered religious traditions that we're broadly familiar with.

There were a wide variety of cults in operation across the Greek speaking world. There were of course adherents to the traditional gods and goddesses, but there were still changes happening and in many places around the Greek world (which was far more expansive than just modern day Greece. Hellenistic king for example introduced numerous new cults. Egypt in particular was a hot spot for the creation or introduction of new deities and cultic practices. Isis, a native Egyptian goddess became widely popular across the Roman world for example she even had a temple at Pompeii! The syncretic deity of Serapis who combined Greek and Egyptian iconography into one deity was also popular. Other cults sprang up around note worthy individuals. Alexander the Great had a royally sponsored cult in Ptolemaic Egypt, and the Ptolemies themselves had a cult for their own family.

There were other Hellenized and Roman faith systems around at the same time. The famous mystery cults of figures such as Mithras, Isis, and other deities proliferated in Late Antiquity, and the Roman Imperial Cult received governmental support up until the conversion to Christianity.

Many of these faith systems and traditions were Greek or at the very least "Greek" and popular across the Greek speaking parts of the Roman Empire (and even penetrated into the Latin West) even if they were a far cry from the stereotypical Greek pantheon and religion. (And this is all without delving into the field of the variously philosophical schools such Epicureanism, Stoicism, Platonism, Neo-Platonism, and so on that were in turn enormously influential on pagan and Christian theology.) These were the religious traditions that Christianity displaced. The version of Greek religion that we grew up learning about never really existed as we often imagine it, and had been undergoing centuries of changes by the time Christianity rolled into town.

By the time of Christianity's ascendancy in the Empire, date it to whenever you'd like the Edict of Milan, the council of Nicaea, the death of Julian, etc... Christianity had already been exposed to numerous other faith systems and in turn been influenced by them. The influence of Manichean faiths, Gnosticism, Platonism, and so on though is beyond the scope of this answer. The point is that there was no switch that was flipped going from pagan to Christian within the Roman Empire, and it was a process that took several centuries, and by the time that the Empire was collapsing in the west, the Church (and I do mean the institutional Church that survives today as the Eastern orthodox Church, the Roman Catholic Church, and the other Eastern Churches such as the Coptic Church in Egypt and the Apostolic Church of Armenia) had more or less formed in recognizable forms. Many modern practices hadn't yet been established, but the ecclesiastical structures, organizations, and so on were at this point incipient and closely tied to Imperial influence.

As for the second part of your question, I've written about this recently, but in short, the answer is no.

There is a lot to go through here, but I will try to make it as understandable as possible.

A major aspect that we need to do away with is the idea that there is any strict division between Christian and non-Christian religions, because the reality of the world is that ancient people and many places throughout history (and to the present day) did not have this singularistic and exclusivist practice of "You are Christian and only Christian." Religions tended to blend together, meld, and syncretize. Thus, Christianity was quite often as pagan as its neighbors, and its distinguishing features was primarily just who they worshiped, but even this is a bit of a misnomer.

In a recent article, for instance, Paula Fredriksen dismisses the idea of a modern conception of monotheism among early Christians and Second Temple Jews, and instead contends that they were definitely polytheistic in some sense of the word, even the apostle Paul. The declaration that the pagan deities were "demons" for instance is not a refutation of their divinity, as a daemon in Greek/Roman faith is still a deity. It is just a particular class of one.

And this kind of melding and syncretism is seen thoroughly, to the point that pagan deities likely found a place within Christian faith, and we have some evidence of this within various "heretical" groups of Christianity of the time. Even going into the Medieval period, we find reference to the gods of the Germanic peoples still being invoked alongside Christ in medical practices and poems. The Aesir, Odin (Woden), and numerous other figures become closely connected to Christian tradition and syncretized, so that we have a Christian-paganism in a way.

In my opinion, I'm not sure there is this particular time when worship just ceased. Instead, it blended and morphed, and probably just syncretized its way into Christian tradition.

A few recommended sources:

Ronald Hutton, The Pagan Religions of the Ancient British Isles: Their Nature and Legacy (Wiley-Blackwell, 1993)

Matthias Gassman, Worshippers of the Gods: Debating Paganism in the Fourth-Century Roman West (Oxford University Press, 2020)

Ross Kane, Syncretism and Christian Tradition: Race and Revelation in the Study of Religious Mixture (Oxford University Press, 2020)

David Frankfurter, Christianizing Egypt: Syncretism and Local Worlds in Late Antiquity (Princeton University Press, 2021)

And there are many others.

(excepting Christianty's appropriation of the tree, etc.)

Since you brought this up, you might also want to check out this answer from u/KiwiHellenist on the subject of "pagan origins of Christmas traditions". While as other posters point out elsewhere, Christianity obviously was influenced by other contemporaneous religions with which it coexisted, Christmas traditions aren't a good example. Pretty much every aspect of modern Christmas is genuinely 100% Christian.