Start with the business truth
Do not begin by importing every spreadsheet. First decide which customer, supplier, product, stock and opening-balance records are authoritative.
Assign one named business owner to approve each data group. Technical success is not enough; the business must confirm that balances and record counts are correct.
Use a controlled migration cycle
Prepare, dry-run, reconcile, correct and repeat. Only schedule the final cutover after the same templates import cleanly and the business signs off the results.
- Freeze the agreed source files
- Reconcile customer and supplier totals
- Validate stock quantities and valuation assumptions
- Record the person who approved each result