I understand how much time it can take to manually un-reconcile your transactions, @Emmerson.

The ability to reconcile your account is a very important aspect of managing your business. However, once there's a mistake, undoing it yourself may pose more problems than a solution. Also, before you can undo an entire Quickbooks reconciliation, you first have to edit every transaction manually.  For more detailed steps, you can read through this article: Undo and remove transactions from reconciliations in QuickBooks Online.

I'd strongly recommend you to consult with an accountant. He or she will be able to undo an entire reconciliation without manually editing individual transactions. This is why only the user's with Quickbooks Online Accountant account has the exclusive ability to undo reconciliation 

Feel free to sing by here in the Community if you have other questions. I'm always here to help.