There are so many stories that have Aziraphale becoming a demon, so, well. You'll see. ^_^
ANGEL:
"Not another word, angel," Crowley muttered at his beaming companion. He could practically hear the angelic choir singing in the background, in time to the sparkles of happiness Aziraphale was giving off. Oddly enough, it didn't put him in a better mood.
NOTORIOUS:
Honestly, he was a notorious demon, evil! Except for, well, you know, being not so very evil, if he was honest about it, which, being a demon, meant he wasn't honest about anything. Anyone would have done it, though. It was just a kid, come on, Aziraphale hadn't been there and he, Crowley, had been and well…
CAR:
So when the kid was being bullied by some kids near the road, and ended up pushed out into said road, in front of a car going at extreme speeds, he couldn't help himself, as stated before, Aziraphale wasn't there to miracle the kid to safety, so, Crowley did.
CLEAN-UP:
He compounded the stupid good deed by catching the boys responsible and, instead of pitching them in front of speeding cars, he made them apologize, and think about their actions, and even gave the speeding driver a ticket and scolding for reckless endangerment. Thinking about it made him ill.
PRESENT:
Which lead to his present problem of an angelically beaming Aziraphale, and a new job, for the second bloody time.
WINGS:
Still smiling, Aziraphale reached out his fingers and touched one of his companion's exposed wings, which for the past however many millennia had been black, but were now a glorious, heavenly in fact, blue. The halo was also a nice touch.
NICE:
Crowley...Er, Camael*, as we must now call him, huffed in disgruntled irritation, this was what he got for being nice, honestly, you did it once** and they expected you to do it all the time, he wasn't made for that, well, he was, but then he wasn't and now he was again… He was also annoyed over the fact that he himself was smiling as brightly, if not more so, as Aziraphale, and was truly rather happy about the whole thing.
RISE:
And thus Crowley became the first angel to fall, and then rise again. His joy knew no bounds.
*According to Sister to the Queen, this means 'he who sees God'
**Actually, he had done lots of good things over the centuries, and all the good had built up until poof! Angelic status reinstated.