Agree the cutover
Choose the last day in the old system and the first day in the new one. Export the original files and keep a read-only copy. Agree whether historical transactions are required or whether opening balances and outstanding documents are enough.
Reconcile before importing
Compare customer ageing to the receivables control balance and supplier ageing to payables. Reconcile cash and banks. List unexplained differences rather than using a balancing entry whose meaning nobody understands.
Prove the result
After the trial import, compare the same reports at the same date and currency. Check a selection of customer balances back to original invoices. Obtain accountant sign-off before new posting. Broad migration and opening-balance import remain planned features in Books.