Diverse Obedience Taken after 12th level

[David knott 242]

At 12th level, a character with the Deific Obedience feat and no other relevant feat, archetype, or prestige class gains the 1st Exalted boon for his deity. So far, there should be no controversy.

But then he decides to take the Diverse Obedience feat at 13th level. Can he now switch out that Exalted boon for the corresponding Evangelist or Sentinel boon, or is he stuck with the Exalted version since it remains a legal choice that he already took?

[PokeyCA]

Because your choices are set when you make them (as confirmed in the Diverse Obedience feat). You wouldn't be able to change it, only be able to pick from the categories on your next pick.