This usually occurs either because the bank recon was not done or done but there were unreconciled transactions still sitting in NS but were not dealt with at the time. These transactions still sitting there will impact the balance in the balance sheet.
Ref: 90689
If the FP has already gone live and all data etc migrated then to correct the bank account balance they could do a spend or receive money transaction on or after the conversion date.
go to the Income bank account (click on it from the Dashboard)
ensure you are on the Account Transactions tab, then click on New Transaction button
ensure the new transaction is 'New Spend Money' if you need to reduce the bank account balance (OR Receive money if you need to increase the bank balance).
enter details to reduce (increase) the balance (diff between conversion & acutal balance). The date has to be on or after conversion date. eg. FP can use the Bank Charges account to isolate the expense. Recommend they consult with thier bookkeeper as to the appropriate account
Save when complete. See below screenshot for suggested details.
then navigate to the transaction, it should be at the top of the list of transactions on the page you are on, click on it. On the transaction page, click on Options in the top right and select Mark as Reconciled. see screenshot below
run the Reconciliation Report for the Income bank account for conversion date and it should have a balance which hopefully agrees with your bank statement