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I'm assuming this is in relation to video games.
If becoming king is treated as an upgrade, that the character's overall abilities are supposed to be changed for the better but you disagree, it might be Power Up Letdown.
If it's treated as more of a horizontal change, that a king is a Supporting Leader and it would be unseemly for a king to run around doing ninja moves as a fighting style anymore, then it might be Gameplay and Story Integration.
you lose you special ability when you became king