Can I undo a combined bank reconciliation- and reconcile monthly will it affect the financial statements cash basis?

Thanks for dropping by the Community, @lathem-joy.

When you reconcile a transaction by mistake, you can remove individual transactions from the reconciliation. Please keep in mind that these changes may result in an unbalance in your accounts. To minimize the impact, you can only unreconcile one transaction at a time.
 

Here's how to edit and unreconcile the transaction:

1. Go to the Accounting menu, then select Chart of accounts.
2. Find the account that has the reconciled transaction. Then select View register.
3. Find the transaction. If the transaction is reconciled, you’ll see an “R” in the checkmark column.
4. Select the transaction to expand the view.
5. In the checkmark column, select the box with the “R” until the box is blank. This removes the transaction from the reconciliation.
6. Select Save.

If you need to undo entire reconciliations, I recommend inviting your accountant to your company. They are the only ones who can undo the reconciliation period at once. 

Once the invitation is done, let your accountant log in as an accountant to access the Undo option. 

To undo a reconciliation, here's how:
 

1. In QuickBooks Online Accountant, select the client from the drop-down list on the Toolbelt to open the client's company file.

2. Select the Gear icon on the toolbar.

3. Under Tools, select Reconcile.

4. On the Reconcile an account page, select History by account.

5. On the History by account page, select the Account and Report period to locate the reconciliation to undo.

6. From the Action column drop-down list, select Undo.

7. When prompted to confirm, select Yes.

8. In the final confirmation window, select Undo.

9. Select Ok.

Moreover, reconciling your transactions will not affect the financial statements in your QuickBooks Online reports. It will only affect your bank account. 

You can also visit this article to learn how to redo the reconciliation: Reconcile an account in QuickBooks Online. 

Let me know if you have further questions about reconciliation by clicking on the REPLY button. I'm always around to help. Keep safe.