"Bring everything across" is what clients ask for, and it is expensive, slow and usually pointless.
Three categories. Must migrate: the open items — open payables, open receivables, the asset register, bank balances, the opening trial balance. Without these you cannot operate. Should consider: master data, meaning suppliers and customers, and prior period balances for comparatives. Usually leave behind: closed transaction history — keep the legacy system read-only for a defined period instead.
The comparatives decision: how many years of prior balances? Two is common, and each year costs effort and adds risk. Ask what the client actually reports on, not what they would like to have.
Master data is a cleansing opportunity. Migrating a supplier master with forty per cent inactive records is migrating a problem. Cleanse before, not after.
The dry run is non-negotiable — full-volume migration into a test environment before UAT. Projects that skip it discover their data problems during cutover weekend, and that is how go-lives slip.
And reconciliation is the deliverable, not the load. Every migrated balance must be signed off against the legacy system by somebody in finance, and the reconciliation pack template gets built during design rather than during cutover.