Fault-Tolerant BMS Modernization in Precision-Controlled Scientific Facilities: Zero-Downtime Migration Architectures

High-precision research facilities demand uninterrupted environmental control during Building Management System (BMS) modernization, yet legacy EcoStruxure platforms with outdated databases and field controllers pose cybersecurity and support risks. Conventional migration approaches relying on planned downtime are incompatible with continuous operations. This paper presents a novel live migration framework enabling zero-downtime transitions while preserving historical trend data and maintaining critical control loops. The methodology integrates parallel infrastructure shadowing, state-preserving database migration, and contingency-driven manual override orchestration to ensure uninterrupted operation of chilled water systems, air handling units, and precision environmental controls. Implementation in a high-precision astronomical facility achieved zero unplanned service interruptions, full preservation of fifteen years of historical records, environmental stability within ±0.008°C, and 99.999% availability during migration. The framework establishes replicable patterns for mission-critical environments—including semiconductor fabs, pharmaceutical clean rooms, and quantum computing centers—providing a validated, risk-free approach to BMS modernization where operational continuity and environmental precision are non-negotiable.