Voices of the Spheres: At 2nd level, an empyreal knight learns to speak and read Celestial, if she could not already. This ability replaces divine grace.
If the character gaining this already speaks Celestial, does that mean it is forced to give up divine grace in exchange for nothing?
In almost all cases, yes. In a home game, I would be willing to let an aasimar (or anyone else already given Celestial for free) another language or a minor but thematic other ability.
Voices of the Spheres: At 2nd level, an empyreal knight learns to speak and read Celestial, if she could not already. This ability replaces divine grace.If the character gaining this already speaks Celestial, does that mean it is forced to give up divine grace in exchange for nothing?
Giving up Divine Grace for nothing, or giving it up for 1 skill point. Not much of a difference. It's still a ricockulously bad deal.
Voice of the Spheres merely represents the single level trade - one that is balanced against the character. Consider the archetype as a whole, though, as that 2nd level trade is only a single piece to the larger pie.