In the 1962 Missal today, for the Feast of the Assumption, you heard the Gospel reading taken from Luke 1: 41-50, recounting the Visitation.
However, in older Missals, this was not the case.
Formerly, the readings were taken from Luke 10: 38-42, recounting the story of Mary and Martha (with the former taking a contemplative, and the latter an active service of Our Lord, and the Lord saying that Mary had chosen the better part).
Here's some pics from my 1937 altar Missal:



Can anyone direct me to a reference/source which explains why this change in Gospel readings was made?