The changeability, a sub-characteristic of maintainability, refers to the level of effort which is required to do modifications to a software product line (SPL) application component. Assuming dependencies between SPL application components and reference architecture implementation (a platform), this paper empirically investigates the relationship between 7 design metrics and changeability of 46 server components of a product line for business applications. In addition, we investigated the usefulness of Platform Responsibility (PR) metric as an indicator of product line component changeability. The results show that most of the design metrics are strongly related to the changeability of server component and also indicate statistically significant correlation between Maintainability Index (MI) and PR metric. The assessment is based on a case study of the implementation of the product line for business applications in a financial institution. The results show that PR metric can be used as good predictor of changeability in the software product line environment.