Validating a Migration When There Is No Legacy to Reconcile Against: A Reference-Value Gate for Consolidating Portfolios into a Bank's Valuation and Position Engine

A bank's daily valuation and position engine values every trading position — mark-to-market, accrual and position — and exports those figures to the downstream systems that build the desks' risk maps and the regulatory reports. It cannot be paused and has no rollback: the close runs overnight and the figures are due each morning. Bringing another entity's trading book into such an engine poses a validation problem that the usual migration safety net does not solve, because there is no comparable legacy engine whose numbers can be reconciled against. This paper reports one such consolidation, carried out from 2011 to 2013 — covering dozens of calculation specializations, several thousand operations valued daily, and a calculation database in the terabyte range — and defines the discipline that made it safe. Its core is a reference-value gate: a functional analyst hand-computes the expected result for each calculation specialization, and the engine's persisted variables must match that reference variable by variable before the specialization enters the official close. Around it sit a specification-first chain, incremental cutover one specialization at a time with two-tier sign-off, and a monitored close with a fix-forward incident loop. In a representative delivery, testing and validation consumed about 47% of effort hours, against 28% for coding. The paper reports what strained the discipline — reprioritization churn and version conflicts, legacy valuation errors surfaced by the migration, and scenarios no specification covered — and relates the reference-value gate to the system-to-system reconciliation gate of an ERP cutover reported separately, as two regimes of one principle: no migrated component enters pratch an objective numeric reference.