A legacy upgrade replaces the parts of a security system that no longer serve the site: unsupported recorders, failing cameras, obsolete credentials. A good upgrade keeps working cabling and housings where they are sound, and stages the change so the building is never left without coverage.
The temptation is to replace everything at once. The risk is that the new system arrives before anyone has documented the old one, and the site loses a week.
What can often be kept
Coax runs can feed encoders, network cabling usually stays, and housings and mounts often survive. Reuse is limited by connector condition and the chosen platform.
Plan the cutover
Photograph the existing layout, record addresses and credentials, migrate cards before retiring readers, and run old and new systems in parallel where coverage is critical.
Related:Commercial securityGlossary: NVR