It's simple: Via castings of the following two spells, the subject may change their feats. The combination of spells is "
Embrace the Dark Chaos" and "
Shun the Dark Chaos" from the "Fiendish Codex I: Hordes of the Abyss" D&D 3.5e supplement, pages 92 and 96 respectively.
Embrace the Dark Chaos:
"The subject immediately gains one Abyssal heritor feat. [...] This Abyssal heritor feat replaces one feat of the subject's choice that it already possesses."
The most important part of the DCFS trick is the effect of the follow-up spell:
Shun the Dark Chaos.
"[An Abyssal heritor feat] is immediately removed and replaced by any other feat for which the subject qualifies. [...] The subject chooses both the feat lost and its replacement."
That's it.
Since writing this, I realize that DCFSing does not allow one to acquire feats for which one does not qualify, as others may otherwise imply.