It was the Resurrection that changed it. As I understand it, the early still Jewish Christians continued to observe the Sabbath (the seventh day), but the also celebrated the "eighth" day -- the first day of the new beginning.
Indeed it was. In the old Hebrew tradition the Sabbath was the last day of the week, the day that God rested. In the Christian tradition the Sabbath is the first day of the week. Sunday doesn't mark the end of anything, it marks the beginning.