I've just migrated from QBD to Online. I connected the bank accounts but then read that this should be done after tidying up the date and reconciling.

Thanks for checking in with us, Julia48.

If there are duplicate transactions downloaded on your Banking page after the data migration, we can exclude the bank transactions. Excluding a transaction means it won’t show up in your account records or financial reports. Before doing this, we can consider the following situations:

• If it’s a duplicate transaction, excluding makes sense.
• If it’s a personal expense, it’s best to add it to QuickBooks instead of excluding it. Otherwise, your bank account in QuickBooks won’t match your bank statement when reconciling. To avoid recording the transaction as a business expense, many business owners add the transaction to an equity account they’ve created, called an owner’s draw.

Once you confirm, just follow the steps in this article: Exclude a bank transaction you downloaded into QuickBooks Online. If there's none, we can disconnect and reconnect your bank account in QuickBooks after tidying up the date and reconciling. When disconnecting an account, there are things we need to take note of: 

• QuickBooks stops downloading new transactions from your bank or credit card account.
• Your existing accounting data and transactions won’t change.
• You can reconnect the account to QuickBooks at any time to start downloading transactions again.
• QuickBooks delete the transactions that still need to be categorised in the For review tab.

For the steps and details, please check out this article: Disconnect accounts connected to online banking in QuickBooks Online. Then, you can connect the bank again once done reviewing your books. To reconcile your accounts so they always match your bank and credit card statements, refer to this article for the detailed steps: Reconcile an account in QuickBooks Online.

If you need further assistance managing your company data, don't hesitate to leave a comment below. I'll make sure to get back to you as soon as possible. You have a good one.