In earlier editions you had the Risen, who could redeem themselves. However, that was a technicality due to the cosmology back then.
In 1E/2E, outsiders were literally made of the essence of their home plane. So fiends (literal elemental evil) were intrinsically evil; becoming good simply wasn't a matter of choice, it implied overcoming their own metaphysical nature, which could only be temporary at best.
Risen (like Fallen) could therefore only come about as a consequence of literal god-level intervention, or spectacularly rare (probably about once per 10,000 years) aberrations where an unambiguously good soul somehow ended up on a lower plane and its goodness remained in spite of its committal. (Occasional souls incorrectly going to the wrong plane wasn't necessarily the rare part, but preserving its own alignment when set against the nature of an entire plane was infinitesimally rarer.)
Rare as it was, though, it still definitely happened, both on the Good/Evil axis and on the Law./Chaos axis (both at once was epsilon-squared unlikely though).
This of course didn't mean fiends couldn't behave good, it just meant that they couldn't become, metaphysically, elementally good.