You're the DM. You certainly can have a fiendish creature be good or a celestial creature be evil. You're the DM. You can do anything you want.
However, speaking as a DM, I think it would be a bad idea.
As I see it, you have things like humans that express the concept of free will. They aren't innately good or evil, but can be monsterous or righteous depending on their choices.
If fiendish and celestial creatures can be monsterous or righteous depending on their choices, then they don't add ANYTHING to your game. You've already got humans, and everything else that is like them - orcs, goblins, elves, whatever - in that conceptual role. All you've done is add another class of humans with bumps on their forehead or wings or whatever.
The natural thinig to assume is that fiendish and celestial beings don't express the concept of free will. They are innately good or evil, and express those concepts by being a literal embodiment of those ideas. In other words, they aren't good because they are celetial, they are celestial because they are good. They are made of good, and not made of some admixture that allows them to choose. Their fundamental nature is good. If for some reason they ceased to be good - a process that is difficult to imagine because it involves replacing their whole body and being - they cease to be celestial and in fact cease to be themselves. In other words, if a celestial being were to become evil, it would also become fiendish and cease to be celestial. Vica versa, a fiendish being that was good, would probably cease to express fiendishness in form as well as nature because FORM IS NATURE in this case. If you want cases were form isn't nature, back up and do humans or something like them (good natured orcs, if you want to play against physical sterotype).
I also generally avoid the concept of 'risen fiend' or 'fallen celestial'. In most peoples common sense understanding of what 'fallen angel' means, it renders the celestial again - little more than a human with wings. That is fundamentally uninteresting and adds nothing to the game not already inherent in 'human'. If you really were interested in the sociology of a society that would develop if humans could fly, have flying humans and explore that. If you are actually interested in good and evil, then don't replace your archetypes for those things with something merely human because it obscures all understanding of the thing studied.