Modernizing a bank’s core systems is one of the most complex and high-stakes undertakings in financial services. As banks move away from legacy technology to unlock agility, resilience, and digital-first experiences, the journey is rarely a straight line. One of the most critical—and often underestimated—phases is coexistence: the period when legacy and new core systems run in parallel. This dual-core state is not just a technical necessity; it is a strategic enabler for de-risking transformation, ensuring business continuity, and delivering value progressively.
Historically, banks attempted “big bang” migrations—moving all products and data to a new core in a single, high-risk event. While this approach may have worked in an era of limited digital engagement, today’s 24/7 banking environment, regulatory scrutiny, and customer expectations make such migrations impractical and risky. The coexistence migration pattern, where legacy and new cores operate side by side, allows banks to phase migrations, test and learn, and minimize disruption.
Coexistence is now the norm for large-scale core modernization. It enables:
But coexistence is not without its challenges. It introduces operational, technical, and organizational complexity that must be proactively managed.
Drawing on Publicis Sapient’s deep migration expertise and lessons from large-scale programs, here are the critical success factors for purposeful coexistence:
Treat coexistence as a core part of your modernization strategy—not a footnote. Plan for it from the outset, recognizing that it will impact every aspect of your technology, operations, and change management. Establish a central coexistence team with the authority and expertise to coordinate across business and technical domains. This team should be distinct from those designing the target state, empowered to make rapid decisions, and supported by senior sponsorship.
Migration (the movement of data and products from legacy to new core) and coexistence (the operational state of running both systems) are interdependent but distinct. The migration team focuses on data extraction, transformation, and loading, while the coexistence team manages the operational, technical, and customer impacts of dual-core operations. Treating these as separate workstreams ensures clarity of objectives and accountability.
Modern architectures make coexistence more manageable. Use your integration layer as the central control point for routing data and transactions between legacy and new cores. The fewer control points you need to manage, the simpler and more robust your coexistence state will be. Consider building a routing layer and a common data layer to aggregate and normalize data for downstream systems, minimizing the impact on reporting, finance, and compliance.
During coexistence, the need to reconcile data across systems increases significantly. Invest early in automated reconciliation capabilities that can compare operational and financial data between cores, flag exceptions, and support regulatory reporting. This reduces manual effort, increases confidence in data integrity, and accelerates issue resolution.
Successful coexistence often requires targeted changes to legacy systems—such as adding migration flags or tags to accounts, or enabling new data extracts. Plan these enhancements early, working closely with legacy support teams and third-party vendors to avoid bottlenecks.
Don’t rely on assumptions about your legacy data. Profile and analyze your full data sets to inform migration tranches and coexistence scenarios. This enables you to define the optimal unit of migration—whether by customer, product, or legal entity—and to minimize complexity and customer impact.
Where possible, pilot coexistence in production with a small set of customers or products. This allows you to validate end-to-end behavior, surface operational issues, and build confidence before scaling up. Early testing is especially valuable in highly regulated environments.
The ultimate goal of coexistence is to fully transition to the new core and decommission legacy systems. Plan the decommissioning process early, aligning it with your migration roadmap and cost-saving targets. This ensures stakeholders remain committed and that the full benefits of modernization are realized.
Publicis Sapient has supported banks of all sizes through coexistence, from global incumbents to digital challengers. Key lessons include:
As banks modernize their core, coexistence is no longer an afterthought—it is a strategic enabler for safe, progressive transformation. By embracing coexistence, empowering central teams, leveraging technology-enabled control points, and investing in automated reconciliation, banks can de-risk their journey, deliver value early, and build the foundation for a truly digital future.
Publicis Sapient brings proven methodologies, deep migration expertise, and a track record of success in managing coexistence for leading banks worldwide. Whether you are just starting your modernization journey or navigating the complexities of dual-core operations, we are ready to help you achieve purposeful coexistence and unlock the full potential of your digital core.
Ready to modernize with confidence? Connect with Publicis Sapient to learn how we can help you plan, execute, and de-risk your coexistence journey.