Charm: I'd make the difficulty (essence x 2) or something. A static one becomes useless at higher levels.

Edit: The problem with the breaking of the castes goes deeper than that really... Why did they break AT ALL? Exaltations are indestructible. The most that can be done is minor changes. (see: abyssals, which are just a flipped switch. And infernals, which are solars with some easily removed yozi gunk) And yet we are expected to believe that every last lunar exaltation was fundamentally BROKEN just by being out in the wyld too long? You know, that place they were created to defend against?