I’m not really sure when it got changed, but I know it’s been since the ’60s, because all the texts I have pre-Vatican II reference that the Feast of the Annunciation is on March 25th. Period. End of story… unless you live in modern America, where date magic and moving immovable feasts has become a cancer of the liturgical calendar. There is a fascination here of screwing with mathematic certainty of when certain events are commemorated: 40 days afters Easter, Our Lord Ascended into Heaven (40 days is exact, given in Luke, Mark, and Acts). 40 days from a Sunday is a Thursday. Hence, Ascension Thursday (not Sunday). It makes sense. Celebrating it 43 days later than Easter does not make sense. There is a consistency being lost because of this tinkering, and Annunciation got the axe this year.
Nine months prior to December 25 is March 25.
Fixing the date of the Incarnation, which occurred at Mary’s acceptance of God’s proposal that she be the Mother of God was a simple matter of common sense. Saint Irenaeus (who lived in the first and second centuries) already wrote of the celebration of the Annunciation on March 25. He went so far as to tie the ideas together: Annunciation is the beginning of salvation, and the passion is its culmination. It followed that the two would coincide. It makes sense that Annunciation and Holy week are intertwined because they are the beginning and the end of the task of Jesus’s work of salvation. Thus, for the feast of the Annunciation to fall during holy week is not a problem, at least not to anyone paying attention.