How do I migrate off a legacy system?

Sequence it so nothing runs twice. Clean identifiers and product data first, since everything downstream depends on them. Move catalog, then inventory, then open orders and purchase orders. Run the old system read-only for a defined period instead of in parallel, because two live systems means dual entry and guaranteed divergence.

The item most often underestimated is open transactions. Orders and purchase orders mid-flight at cutover have to land in one system, and deciding which before the date is easier than adjudicating afterwards.

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