Purposeful Coexistence in Your Journey to a Digital Core: Best Practices for Managing Dual-Run and Migration Complexity

Modernizing a bank’s core is no longer a question of if, but how. As financial institutions move away from legacy technology to unlock agility, resilience, and customer-centric innovation, the journey to a digital core is defined by one of its most challenging phases: coexistence. This is the period where legacy and new core systems run in parallel—a dual-run state that, if not managed purposefully, can introduce significant risk, operational complexity, and cost. Yet, when approached with the right strategies, coexistence becomes a powerful enabler for safe, progressive transformation.

Why Coexistence Matters

Historically, banks attempted “big bang” migrations—moving all products and data to a new core in a single, high-stakes event. While this approach was once feasible, today’s 24/7 digital banking expectations, regulatory scrutiny, and the batch-based nature of legacy systems make such migrations risky and often impractical. The industry has learned, sometimes painfully, that coexistence—running old and new cores together for a period—is the safer, more effective path. It allows for phased migration, controlled risk, and the ability to test, learn, and adapt in real time.

The Complexity of Coexistence

Coexistence is not a simple technical bridge; it is a multi-dimensional challenge that touches every part of the bank. It requires careful orchestration of data, processes, technology, and people. Too often, coexistence is treated as an afterthought—just a technical hurdle to clear on the way to new capabilities. In reality, purposeful coexistence must be planned as a first-class concept, with dedicated leadership, investment, and a clear operating model.

Transition States: A Phased Approach

A successful coexistence strategy recognizes that the journey from legacy to digital core happens in stages:

Each transition state requires its own technical, operational, and organizational strategies.

Best Practices for Managing Coexistence

  1. Treat Coexistence as a Program, Not a Project
    Establish a central, empowered coexistence team with senior sponsorship. This team should be distinct from the migration and business-as-usual teams, with a mandate to make cross-cutting decisions, manage dependencies, and drive holistic change management.
  2. Separate Migration from Coexistence
    Migration is the act of moving data and products from legacy to new core. Coexistence is the state in which both systems operate together, often for months or years. Each has distinct objectives and requires different expertise. Don’t expect your migration team to own coexistence end-to-end—plan for both, and ensure clear accountability.
  3. Build a Common Data Layer
    A unified data layer that aggregates information from both cores and feeds downstream systems (e.g., general ledger, reporting) is essential. This minimizes disruption, ensures data consistency, and enables seamless operations during the coexistence phase.
  4. Create a Routing Layer
    A robust routing layer determines which core system should process each transaction or request. This layer, often supported by a data cache, enables efficient, transparent routing and reduces the need for complex changes in customer or product systems.
  5. Invest in Automated Reconciliation
    With data flowing between multiple systems, reconciliation becomes exponentially more complex. Invest early in automated, repeatable reconciliation processes that can handle operational and financial data across both cores. This is critical for regulatory compliance, customer trust, and operational efficiency.
  6. Tactically Invest in Legacy Systems
    Some changes to legacy systems are inevitable—such as adding flags to indicate migrated accounts or enabling new data flows. Plan these changes early, and coordinate closely with legacy support teams and third-party vendors.
  7. Go Deep on Data Analysis
    Don’t rely on assumptions about your legacy data. Profile and analyze your full data sets to inform migration tranches and minimize coexistence complexity. This analysis will help determine the optimal unit of migration—by customer, product, or other attributes.
  8. Test and Prove in Production Early
    Where possible, pilot coexistence with a small set of customers or products in the live environment. This de-risks the transition, surfaces unforeseen issues, and builds confidence for broader rollout.
  9. Plan for Decommissioning from Day One
    The ultimate goal is to retire the legacy estate and realize the full benefits of modernization. Plan decommissioning activities early, track progress, and align stakeholders on the cost savings and value realization timeline.

Lessons from Real-World Programs

Banks that have succeeded in core modernization share several common traits:

For example, leading banks have built dedicated coexistence control centers, established common data and routing layers, and used automated reconciliation to ensure data integrity throughout the transition. These approaches have enabled them to migrate at pace, minimize customer disruption, and accelerate the decommissioning of legacy systems.

The Mindset Shift: Coexistence as a First-Class Concept

Perhaps the most important lesson is cultural: coexistence is not a technical footnote, but a strategic enabler of safe, progressive transformation. Treating coexistence as a first-class concept—worthy of leadership attention, investment, and rigorous planning—sets the foundation for a successful journey to a digital core.

Ready to Navigate Your Coexistence Journey?

Publicis Sapient brings deep experience in designing and delivering coexistence strategies for banks of all sizes. Our proven methodologies, technology accelerators, and industry partnerships help clients de-risk migration, accelerate value realization, and build the digital core needed for future growth.

Let’s work together to make coexistence a catalyst—not a constraint—on your path to digital banking leadership.

For more information on how Publicis Sapient can support your core modernization journey, visit publicissapient.com.