Managing changes in source system terms and surveilling for associated deviations in HL7 reporting is an essential, but difficult aspect of a health information exchange. We analyzed the mapping records of the Indiana Network for Patient Care in order to characterize the evolution of radiology and laboratory system terms after initial implementation with regard to term mappings and changes in units of measure. Overall, we added half as many new post-implementation terms (9909) as we added for initial system implementations. As a group, INPC institutions have not slowed much in their rate of adding new terms after initial implementation. In general, we encountered unit-related exceptions less frequently than new, unknown terms. Our study highlights the ongoing effort required to keep up with evolving source system terms in a regional HIE and the need to willingly embracechange along the way.