Effective mapping and retrieval are important issues in successful deployment of plan reuse strategies. In this paper we present a domain independent strategy for ranking a set of plausible reuse candidates in the order of cost of modifying them to solve a new planning problem. The cost of modification is estimated by measuring the amount of disturbance caused to the validation structure of a reuse candidate if it were to be reused in the new problem situation. This strategy is more informed than the typical feature based retrieval strategies, and is more efficient than the methods which require partial knowledge of the nature of the plan for the new problem situation to guide the retrieval process. We discuss the implementation of this retrieval strategy in PRIAR, a framework for flexible reuse and modification in hierarchical planning.