Let's say a Pit Fiend turn its own spell-like ability, fireball, into a supernatural ability with the feat "Supernatural Transformation".
And before doing so, he has also taken Quicken Spell-Like Ability (Fireball).
Since the Fireball is now Supernatural Ability, does that mean he can no longer quicken it, and the feat Quicken Spell-Like Ability (Fireball) becomes completely useless for him now? Or can he still quicken it since it is originally a spell-like ability?
If a pit fiend doesn't have the spell-like ability fireball anymore because that pit fiend took the feat Supernatural Transformation (fireball) (Savage Species 39) so as to change its spell-like ability into a supernatural ability, then that pit fiend made an unwise choice when it earlier picked the feat Quicken Spell-like Ability (fireball) (Monster Manual 304). It can't apply to a supernatural ability a feat that must be applied to a spell-like ability.
The pit fiend would need to take something like the feat Quicken Supernatural Ability (that doesn't exist in the official corpus so far as I'm aware) to employ its supernatural ability fireball as a swift action.
A pit fiend, though, has the spell-like ability wish usable once per year, so if a pit fiend did make such a mistake—taking the Quicken Spell-like Ability (fireball) feat then obviating it by taking later the Supernatural Transformation (fireball) feat—, then (at worst) it could employ a wish to fix it.
