Lauren Faust explains it is like the DBZ effect. She starts out with barely any power, so she has her regular mane. But when she as she gains más power, she grows and gets the ethereal mane.
You're an Idiot If she didn't revive the mostrar with her ideas it might have remained as the G3 version and even if someone came and revived it it wouldn't be the same.
I think that, during the daytime, her mane is the gentle, sea blue mane that we first saw her with. During the night, it reverts back to Nightmare Moon-like starry mane, when she has control.
Her mane probably changed of aging. She is a fully grown alicorn now just like Princess Celestia. Princess Cadence might not be a fully grown alicorn, but she will sooner.