Before revision: PD curve calibration refers to the task of transforming a set of conditional probabilities of default (PDs) to another average PD level that is determined by a change of the underlying unconditional PD.
After revision: PD curve calibration refers to the task of transforming a set of rating grade-level probabilities of default (PDs) to another average PD level that is determined by a change of the underlying unconditional PD.