It'd most likely be classified as a celestial
Outsiders like devils, demons, and angels seem to work on a weird sliding scale of alignment. If you go from one alignment to the other, you essentially abandon your old classification and take up a new one. Even your statistics might change because your very fabric of existence has changed.
Zariel was once a lawful good angel. She fell from grace because she wanted to intervene in the Blood War to aid the devils, and her alignment became lawful evil. She became a devil, and later an arch-devil.
She can also be redeemed in a new campaign book, in which case she becomes an angel again.
Azael was once a fallen lawful good angel, but he was punished for his evil deeds and sent to the Abyss, where his alignment became chaotic evil. He's now classified as a demon.
Graz'zt (thanks for reminding me of his name, @NathanS) was a devil lord who betrayed Asmodeus because he tired of all the rules and went rogue, changing his alignment from lawful evil to chaotic evil. He's now classified as a demon and he's one of the demon lords of the Abyss.
So to answer your question, your devil in question would now be a Chaotic Good outsider. The hard divide between lawful good, chaotic good and neutral good in terms of Celestials is not quite as defined as it is for demons and devils, and I don't believe there's a generic overarching name for chaotic good celestials (unlike lawful good, which tend to be called angels), so 'celestial' would likely be the most likely new classification after drawing the card.