We present a theory of plan modification applicable to hierarchical nonlinear planning. Our theory utilizes the validation structure of the stored plans to yield a flexible and conservative plan modification framework The validation structure, which constitutes a hierarchical explanation of correctness of the plan with respect to the planner’s own knowledge of the domain, is annotated on the plan as a byproduct of initial planning. Plan modification is characterized as a process of removing inconsistencies in the validation structure of a plan when it is being reused in a new (changed) planning situation. The repair of these inconsistencies involves removing unnecessary parts of the plan and adding new non-primitive tasks to the plan to establish missing or failing validations. The resultant partially reduced plan (with a consistent validation structure) is sent to the planner for complete reduction. We discuss the development of this theory in PMAR system, and characterize its completeness, coverage, efficiency and limitations.