Migrating from Legacy EPM to OneStream: A Practical Guide

If your organization is still running Oracle Hyperion, SAP BPC, or IBM Cognos for financial consolidation and planning, you're likely feeling the pressure to migrate. With SAP BPC end of support set for 2030 and Oracle's Hyperion roadmap increasingly focused on Cloud EPM, the window for a well-planned migration is narrowing. Legacy EPM platforms served their purpose, but the cost of maintaining them -- both in infrastructure overhead and in the fragmented workflows they impose on finance teams -- is becoming harder to justify. This guide covers what we've learned helping organizations make the move to OneStream, from scoping and data strategy through go-live and adoption.

Why Organizations Are Moving Now

The migration trend is not speculative. Over 70% of OneStream's 1,700+ customers replaced multiple legacy applications when they moved to the platform. The drivers behind these decisions are tangible: aging on-premises infrastructure that IT teams are increasingly reluctant to maintain, a lack of meaningful vendor investment in legacy product development, and fragmented toolsets that require manual data movement between planning, consolidation, and reporting systems. OneStream replaces all of it in a single unified platform -- one data model, one security model, one set of business rules. Organizations that have made the switch report average improvements of 39% in financial close cycle times, 74% in reporting efficiency, and 32% in planning and budgeting processes. These aren't aspirational numbers; they reflect the operational gains of eliminating the integration tax that legacy EPM architectures impose.

Scoping the Migration: What to Bring, What to Leave Behind

The single biggest mistake we see is treating a migration as a lift-and-shift. Legacy EPM systems accumulate years of workarounds, unused reports, and redundant hierarchies. A Hyperion instance that has been in production for a decade will have dozens of data forms nobody opens, reporting trees that reflect organizational structures from three reorganizations ago, and calculation scripts with logic that no one on the current team fully understands. Start by defining what actually matters: which reports are actively used by finance leadership, which hierarchies reflect the current legal and management structure, and how much historical data you actually need for comparative reporting and statutory disclosure. Migrating less data with stronger intent consistently produces better outcomes than attempting to replicate everything. This is your opportunity to rationalize, not just relocate.

Dimension and Hierarchy Alignment

Entity and dimensional hierarchy mapping between your legacy system and OneStream is the most critical technical workstream in the entire migration. This is where the structural differences between platforms become real. Intercompany elimination logic, currency translation rules, ownership structures, and metadata naming conventions all need careful validation against OneStream's dimensional model. Account hierarchies may need restructuring to take advantage of OneStream's unified chart of accounts approach. If your legacy system uses separate hierarchies for consolidation and planning, you'll need to reconcile those into a single structure. Getting this wrong creates downstream issues in elimination processing, currency translation, and report output that are expensive to fix after go-live. We recommend dedicating a focused workstream to dimension design with sign-off from both the technical team and the finance stakeholders who own each hierarchy.

Data Migration Strategy

Define your historical data scope by business use, not by completeness. The instinct to migrate everything is understandable but counterproductive. Statutory disclosure requirements typically require 2-3 years of comparative data. Management reporting may need more for trend analysis, but that data can often be served from a data warehouse rather than loaded into OneStream. Once you've defined the scope, build your migration pipeline with validation checkpoints at every stage: source extract verification, transformation audit, load confirmation, and reconciliation against the legacy system. Run parallel periods where both systems are active and producing output for the same reporting cycles. Parallel testing is non-negotiable -- it is the only reliable way to confirm that the new system produces numbers your finance team can trust. Plan for at least two full close cycles in parallel before decommissioning the legacy platform.

Timeline and Phasing

A typical Hyperion or BPC to OneStream migration runs 4-6 months depending on complexity and organizational readiness. That timeline assumes a phased approach, which is what we recommend in nearly every engagement. Phase 1 covers financial consolidation and close -- the core use case that most organizations need to get right first. Phase 2 adds planning and budgeting, extending the platform into forward-looking processes. Phase 3 brings in reporting, analytics, and any specialized solutions like account reconciliations or transaction matching. Trying to do everything at once increases risk across the board. A phased rollout lets you validate each capability with real users and real data before adding the next layer of complexity. It also gives your finance team time to build confidence in the platform incrementally rather than facing a wholesale process change on day one.

Change Management Is the Hard Part

The technology migration is the straightforward part. The real challenge is getting 50-200 finance users to adopt new workflows, trust new numbers, and let go of the Excel-based workarounds they've relied on for years. This is not a training problem you solve with a two-hour webinar the week before go-live. Start user training early in the project, ideally during UAT when users can interact with their own data in the new system. Identify power users in each department and involve them in testing and validation -- they become your change champions and the first line of support when their colleagues have questions. Build reference materials that map old workflows to new ones so users can see exactly how their daily tasks translate. Organizations that underinvest in change management consistently struggle with adoption even when the technical implementation is sound. A system that finance users don't trust or don't use delivers no value regardless of how well it was built.

Common Pitfalls We See

After supporting multiple legacy-to-OneStream migrations, certain patterns of failure repeat themselves. Underestimating data cleanup effort is the most common -- plan for data remediation to consume roughly 30% of your total project timeline. Skipping parallel testing periods is another frequent shortcut that leads to post-go-live reconciliation crises. We also see organizations try to replicate their legacy processes exactly instead of redesigning for OneStream's architecture, which defeats the purpose of moving to a unified platform. IT security teams need to be involved early in SaaS deployment planning, particularly around SSO configuration, network access, and data residency requirements -- these conversations take longer than expected and can delay go-live if left until the end. Finally, document your business rules and calculation logic thoroughly before decommissioning the old system. Once that legacy platform is turned off, the institutional knowledge embedded in its configuration becomes inaccessible.

Final Thoughts

The migration window is real. SAP BPC's 2030 end-of-support date and Oracle's strategic pivot to Cloud EPM mean that organizations still running on-premises legacy EPM platforms will face increasing maintenance costs and decreasing vendor support. Organizations that start planning now have the advantage of choosing their own timeline instead of being forced into a rushed cutover driven by external deadlines. A well-executed migration to OneStream doesn't just replace your legacy EPM -- it eliminates the fragmentation that has been slowing your finance team down for years, consolidates your technology footprint, and gives you a platform that can grow with the business. The organizations that treat this as a strategic initiative rather than a technical project are the ones that get the most out of it.