ANSWER
How do you retire a legacy system without stopping operations?
Reviewed .
To retire a legacy system without stopping operations, do not cut first. Run the replacement in parallel with the current application, feed both the same inputs, compare their outputs for an agreed period and cut over only after reaching the acceptance threshold defined in advance. The main risk is discovering too late that the old application performed undocumented work.
Step by step
- Inventory actual behavior from usage, not the manual.
- Map business rules hidden in code and team practice.
- Shadow-run the replacement with no production effect.
- Classify every variance before cutover.
- Cut only after the agreed threshold, with a written rollback plan.
How an agent does this
AgenticosCore calls this Parallel. It is mandatory for size G. Every shadow execution is recorded in the Trail; the cutover remains your decision and the Brake can stop the agent immediately.
Frequently asked questions
How long should the Parallel run last?
Long enough to cover a full process cycle, including close and seasonal exceptions. The match criterion, not the calendar, decides when to stop.
What if the old system is wrong?
That is a common and valuable finding. The variance is classified so the old error is not blindly reproduced.
Primary sources
- Retiring service components securely — UK Government Security