Hello there, hmacattack.

Undoing a reconciliation in QuickBooks Online will not erase individual transaction categorizations, memos, and attachments. However, it'll unreconcile previously reconciled transactions and reverse any changes made during the reconciliation process.

Additionally, undoing the entire reconciliation is only available in QuickBooks Online Accountant. If you have a different version, I suggest contacting your accountant. They can undo the entire period at once.

Here's how:

1. Go to the Gear icon.
2. Select Manage Users.
3. Click the Accounting Firms tab and select the Invite button.
4. Enter the accountant's information.
5. Tap Save.
6. Let your accountant undo the entire reconciliation.

If you prefer to do it by yourself, follow these instructions to undo reconciled transactions one at a time:

1. Go to the Transactions menu. Then select Chart of Accounts.
2. Find the account and select View Register. 
3. Filter the register.
4. Review the check column. If the transaction is reconciled, you’ll see an R.
5. Click the box repeatedly until it's blank to remove the transaction from the reconciliation.
6. Hit Save. Then close your register.

You can check this article if you encounter reconciliation issues: Fix issues when you're reconciling accounts.

Please come back if you need any further assistance with undoing the reconciliation. The Community is always here to help.