it says that Satan transformed himself into an angel of light.
If he transformed himself, it means that was not his original state..
I think the remarkable thing here is the transforming that makes him appear not as an ANGEL but as an angel OF LIGHT. He is an angel already but not of LIGHT.
Those singing in heaven could be the redeemed that are already there--David Jeremiah doesn't believe angels sing, but I disagree--I believe they do.