This article from the Healthcare Informatics website discusses the challenges of migrating from the ICD-9 code set to ICD-10 code set. This results primarily from the added complexity and granularity of the ICD-10 code set, which means that a specific ICD-9 code may map to multiple ICD-10 codes (since the ICD-10 codes contain additional information not contained in the source ICD-9 code).

Toward the end of the article, the author mentions an online tool to gauge the mapping impact of a specific set of ICD-9 codes to their ICD-10 equivalents. This is a useful tool for providers who typically use a subset of ICD-9 codes for their current practice. The link is not mentioned in the article, but a bit of searching found it here.

This is a good read for individuals tasked with performing an ICD-9 to ICD-10 impact assessment.