Legacy System Modernization with Coding Agents: A Case Study

Legacy systems built on discontinued platforms are a recurring technological liability in organizations that depend on these applications to sustain critical business processes. Although modernization is strategically necessary, it is costly and error-prone when performed exclusively through manual effort. In this paper, we report on a case study conducted in a real industrial setting, where we evaluate both the effectiveness and efficiency of AI coding agents in supporting the migration of legacy systems to modern platforms. By using one version of Claude Code agent, we migrated 12 features of distinct complexity levels from a corporate ERP system written in Visual Basic 6 to C# .NET 10 under a single-generation strategy. Once the migration sessions were completed, we measured the equivalence of the migrated features against the original ones, and collected data about the time spent and the number of tokens consumed by the agent during the process. In our experiment, the agent achieved an average equivalence of 70%, with a strong asymmetry across complexity levels; low-level features achieved 92%, and high-complexity ones scored 47%. Similar asymmetry was observed in the cost-based metrics; low-level features consumed 1.47M tokens (1.66), whereas high-level ones used 9.09M (10.28). We reveal the practical scenarios and circumstances where this migration strategy is more effective, as well as discuss the limitations and challenges of using AI agents for legacy system modernization.