A cost system provides a snapshot of a firm’s resource consumption by cost objects. However, the firm’s operating environment and the firm itself change over time, affecting the costs to be allocated and resource consumption patterns. A firm may wish to update its cost system to capture these changes. We define various levels of thoroughness in cost system updating, and study the benefits that result when choosing a product portfolio based on data from the cost system. We find that the benefits of partial updates (e.g., updating overhead rates) are limited, and on average result in a reduction in profit relative to not changing the system at all. Further, implementing a sophisticated cost system in the first place results in higher profit than a thorough update later. Lastly, the benefits of a thorough update are highest when the impact of the change in the environment is the greatest.