Original cover art from HERE

Despite getting over her fears about her Cutie Mark, Apple Bloom is still having nightmares about whether she and her friends will stay together once they're no longer Crusaders.

To quell this fear, once and for all, Princess Luna visits Apple Bloom at Sweet Apple Acres and casts a spell that should allow Apple Bloom to see how Luna dealt with her own fears regarding her Cutie Mark when she was younger from her own perspective.

Something goes wrong, however and Luna does more than let Apple Bloom see what it's like to be the Princess of the night, she literally becomes it herself, becoming an Alicorn, growing to Luna's size, gaining her Cutie Mark and her mane and tail become ethereal and dotted with stars, whilst Luna is reduced to a Blank Flank Earth Pony filly.

Now they must figure out if the spell can be reversed, whilst Apple Bloom takes on Luna's duties and Luna must learn to live as a filly.

However, Luna's spell does as intended. During her patrols of the Dream Realm, Apple Bloom will learn things about certain ponies that she never could've guessed, nor how it will effect her.

In Popular Stories 17/7/2015. Didn't actually expect that.