Prior to the angels being cast out of heaven, in Paradise Lost by John Milton, the angels changed. Prior to being cast out, the angels were bright and shining.
In the happy realms of light
Clothed with transcendent brightness, didst outshine
Myriads, though bright!
After being cast out of heaven, the angels were required to exist in darkness.
No light; but rather darkness visible,
Their brightness has been taken away.
Though changed in outward lustre.
Essentially, the angels were no longer able to appear as the glowing and shining beings they were in heaven. Instead, thrown into darkness, their brilliant light was extinguished as they were forced to live in the "mournful gloom" of Hell. Therefore, after being exiled from heaven, the angels are no longer able to wear their brilliant light (as they had in heaven).