Story behind Upakarma(sacred thread change)

Upakarma is usually held on the full moon day of the month of Sravana. The significance of this day is that Lord Vishnu took the form of a horse and restored the Veda that was stolen from Lord Brahma by the demons Madhu and Kaitabha. As lord Vishnu took the form of a horse, this incarnation is called Hayagriva or "horse-head". 

After lord Vishnu created lord Brahma, he taught Brahma the all eternal Vedas. Once lord Brahma had mastered the Vedas, he was filled with pride that he was the only entity that had the knowledge of the all eternal and holy Vedas. Lord Vishnu thought otherwise and created demons Nectar and Kaitabha from two water drops on the lotus that he mounts. He then instructed them to steal the Vedas from Lord Brahma and hide it. Thus, Lord Brahma was in a fix that he was not able to save the holy and all eternal Vedas from theft and prayed to Lord Vishnu to do the needy. Lord Vishnu took the form of Hayagriva or Hayavadana and restored the all pervading Veda to safety,.[1] thus curbing the pride of Brahma. 

So the day of upakarma is also celebrated as Hayagriva utpatti. As the Vedas were restored on this day, Upakarma is performed on this day to mark a new beginning.