Even a small error in an accounting migration haunts you all year, because every statement rests on the opening balance. Here is the sequence we use with every customer.
Lock the legacy period
Pick a clear cut-off, such as period-end, and post nothing more in the old system after it.
Book opening balances as one entry
Take the trial balance at the cut-off date, post it as an opening journal, then reconcile account by account against the old books.
Migrate sub-ledgers separately
AR, AP and fixed assets move line by line, with due dates and accumulated depreciation.
Run parallel for one period
Post in both systems for a month and match the statements before retiring the old one.
Keep a read-only copy of the old system
At least five years, for later audits.
