User blog comment:B214/Question and Answers 6/@comment-180.253.126.123-20160717050337/@comment-24.113.252.213-20160717061457

Well, we know that their soul stays the same. That's how those reincarnated keep any special powers or characteristics they had originally, like the Light Attribute that allows humans to wield Holy Swords, whatever allows humans to wield Demon Swords, and Sacred Gears. Even powers and characteristics unique to their original race/species (Youkai, Fallen Angel, Dragon, ect.) are kept.

So if it's not the soul that's changed, then it's likely that the Evil Pieces/Brave Saints change the "physical body" into that of a common Devil/Angel, but with the physical appearance it originally had.

For example: Tannin. He was originally a Dragon, so he is now a Devil that looks like a Dragon, and has the abilities of a Dragon. He's a "Dragon-Shaped-Devil."

We could think of the Reincarnated Devils/Angels as an "Artificial Sub-Species" of the Devil/Angel race.