Start with a business and technical reason
Tie the move to a concrete outcome: support for a target operating system, new administration and security controls, PostgreSQL improvements or compatibility with upcoming application releases. A version number alone is not a success criterion.
Map dependencies
Inventory application configurations, extensions, external reports, device drivers, cryptographic components, web publications, scheduled jobs and integrations. Record the exact versions of clients, application servers, databases and operating systems.
Rehearse a working day
Opening a database is not an acceptance test. Use a representative copy to run document posting, period close, printing, banking and EDI exchanges, scheduled jobs and heavy reports. Evaluate the new 8.5 interface as a separate adoption stream so usability feedback is not mixed with compatibility defects.
Design rollback before deployment
Set an upgrade window, owners, checkpoints and a maximum interruption time. Prove that the backup can be restored, preserve the previous binaries and publication settings, and define a clear go/no-go point. Completion means a stable control period, not merely a successful first login.