Board Thread:Lore Discussion/@comment-27673129-20170414055455/@comment-32089425-20170524190515

The difference between the Aedra and the Daedra is that the Daedra were the ones who turned down Lorkhan's plan to create the world. When Lorkhan proposed the creation of the world, Merid-Nunda, who would later become known as Meridia, was one of those who agreed to participate, making her an Aedra.

All Magna Ge were once Aedra. When Lorkhan's true intentions were revealed and Mundus began stealing essence Magnus and a bunch of other Aedra, fled Mundus to Aetherius through Oblivion to preserve their power, creating the stars as a biproduct. The Aedra who followed Magnus were termed Magna Ge. Merid-Nunda was among these, making her a Magna Ge.

Later, Merid-Nunda began consorting with Daedra and was cast down, becoming Meridia, the Daedric Prince of Life and Lady of Infinite Energies, making her a Daedra.

This is the only time (that I know of) that an Aedra ever became a Daedra in this way.

TL;DR: Meridia used to be an Aedra, she became a Magna Ge after following Magnus away from Mundus. She became a Daedric Prince after consorting with the Daedra.