Hi there, @dengel. Allow me to share some insights about your query.

I understand that having the option to restart/undo a reconciliation can save a lot of time.

As of the moment, here’s what you can do. You can manually undo a completed reconciliation period, one transaction at a time. Another option is to have an accountant user undo the whole period.

To manually edit, these are the steps.
 

1.  Go to the Gear menu and choose Chart of Accounts.
2.  Look for the bank where you're reconciled transactions belong.
3. Click Account history.
4. From there, you'll see reconciled transactions marked as R under the Check column. 

 5.To undo them, you have to repeatedly click the box to change the status to Blank (unreconciled). 
 6.Once done, select Save.

For more information, you can refer to this article about how to undo or remove transactions from reconciliations in QuickBooks Online.

In case the reconciliation is still on progress, you can click the Resume reconciling and then Edit info.

For visual reference, you can refer on the screenshot below. 

If there’s anything else I can do to assist you with undoing your bank reconciliation, please post a reply below. I’m always available to help if you ever need one. Keep safe!