Rehost, Refactor or Rewrite: How to Choose the Right Mainframe Modernisation Path

Mainframe modernisation is not a one-size-fits-all decision, with organisations typically choosing between rehosting, replatforming, refactoring, rearchitecting, replacing, or retiring legacy systems. Each strategy carries a different cost, timeline, and risk profile, making the choice highly dependent on specific business drivers such as cost reduction, agility needs, and available skills. A key pressure across UK enterprises is the shrinking pool of COBOL and mainframe expertise, alongside high operating costs and difficulty integrating legacy systems with modern cloud and AI platforms. Rehosting is the fastest and cheapest option but does little to address underlying code problems, while refactoring — often involving automated COBOL-to-modern-language conversion — is considered the sweet spot for most organisations with sound but hard-to-maintain logic. Most large modernisation programmes blend multiple strategies across their application estate rather than applying a single approach uniformly.