Parallel runs without doubling the panic

A parallel run is not a vague “transition period.” It is a controlled overlap where the old chain still feeds the business and the new Python path proves it matches — or fails loudly.

Assign a comparer

Someone must reconcile outputs daily. If nobody owns the comparison, you only added work.

Set an exit rule

Example: five consecutive matching mornings, or three matching mornings plus one supervised failure recovery. Write the rule before day one.

Expect vendor noise

Exports rename. Columns shift. Budget calendar slack. In Taichung we have seen rainy-week vendor changes slip a cutover; the runbook absorbed the new failure name afterward.

Utility Maple builds parallel runs into operations automation engagements because cutover confidence beats theatrical launch dates.