The original change was before the current canon law of 1983. They give the exceptions for 1) Saturday morning plus evening Mass, 2) Holy day of obligation morning plus evening Mass 3) Holy Thursday morning plus evening Mass 4) Easter second Mass and 5) Christmas Midnight plus day Mass.
It was first changed in
Immensae Caritatis —On Facilitating Reception Of Communion In Certain Circumstances, Sacred Congregation of the Sacraments, January 29, 1973
- A More Extensive Faculty To Receive Communion Twice In One Day
The discipline now in force permits the faithful to receive communion a second time on the same day:
—on Saturday evening or the evening before a holy day of obligation, when they are fulfilling the obligation to assist at Mass, even if they have received communion that morning;3
—at the second Mass of Easter Sunday and in one of the day Masses on Christmas, even if they have received communion at the Mass of the Easter Vigil or at the Mass at Midnight on Christmas;4
—also at the evening Mass on Holy Thursday, even if they have also received communion at the chrism Mass.5
Over and above those listed, there are other situations of the same type that favor a second communion.