The definition of the Immaculate Conception has not changed, since it wasn't dogmatically defined until 150 years ago. However, the belief that Mary was conceived without the stain of original sin dates from the time of the Church Fathers, and the it was debated over by medieval theologians, until Duns Scotus resolved the theological debate.
"However, the belief that Mary was conceived without the stain of original sin dates from the time of the Church Fathers, and the it was debated over by medieval theologians, until Duns Scotus resolved the theological debate."
Not quite. The Orthodox East has never accepted the concept of the Immaculate Conception as advanced by the Roman Church because the Eastern Church does not ascribe to the Western Augustinian notion of "Original Sin" nor, consequently of any "stain" from it.
The Orthodox Church, in conformity with the Fathers of the Church, teaches that the Theotokos, from the moment of her birth was free from all sin and remained that day until the Dormition and Assumption. We celebrate the Conception of the Most Holy Theotokos on December 9. It is a major feast of the Church. Here are the Apolytikion and Kontakion from the Feast Day Liturgy:
Apolytikion:
Fourth Tone
Against all hope, the bonds of barrenness are loosed today. For, God has hearkened unto Joachim and Anna clearly promising that they would bear a godly maiden. He who commanded the angel to cry out to her, "Hail, full of grace, the Lord is with you," will be born of her, the infinite One Himself, becoming man.
Kontakion:
Fourth Tone
Today the world rejoices in the conception of Anna, wrought by God. For she bore the One who beyond comprehension conceived the Logos.