Hi there, @MLH114. I can share details about how QuickBooks Online (QBO) works in changing posting accounts for product and service items.

Upon changing the income or expenses account associated with your product and service item, there's no way we can select a specific period where we can apply the changes retroactively. We can either select the Also update this account in historical transactions box and manually delete then re-create the transactions you want to keep from its original income/expenses posting account. Otherwise, we can change the account section and apply changes for future transactions. 

To void or delete transactions, please follow the steps below:

1. Go to the Sales menu and select All Sales or Expenses.
2. Locate the transaction, and select Void or Delete by drilling down the small arrow under the Action column.
3. Select Void/Yes or Delete/No to confirm.
4. The other way to do that is to go to the required transactions and select View/Edit by drilling down the small arrow under the Action column.
5. Enter the additional information you want to include in deleting or voiding the transactions.
6. In the footer section, click More and select Void and Delete.
7. Tap Yes to confirm.

Moreover, you can visit this article to see your best sellers, what’s on hand, the cost of goods, and more: Use reports to see your sales and inventory status. 

You’re always welcome here in the Community if you have more concerns with managing inventory items or any QuickBooks topics. Stay safe!