Sold non inventory items that were coded incorrectly and now do not show up as Cost of Goods Sold. How to correct past items?

Transactions associated with Non-inventory items don't reflect the Cost of Goods Sold (COGS) in QuickBooks Online (QBO), @Andrew142. Let me share why.

Once you sell a non-inventory item, it'll only affect the income account. If you associate COGS as your Expense Account for your non-inventory item, this will only affect the account when you purchased this item.

On the other hand, when updating invoices, note that it won't impact invoices with non-inventory items as these are expenses at the time of purchase. 

The best thing to do is adjust the bills used to purchase the non-inventory items or create a Journal Entry. However, creating journal entries requires an accountant's advice for accurate financial data.

Furthermore, you can personalize your reports in QBO and focus on the areas that matter to your business.

I'll be here to help with any other questions about non-inventory items and COGS. Take care.

@CarlSJ 

"I'll be here to lend a hand if you require any other queries with reflecting sold non-inventory items as COGS. "Take care."

Your customer asked how to fix an issue which you completely failed to address.  Are you AI?  

@Andrew142 

"Do I need to change it from Parts to something else then back again? It seems that there would be an easy to do this."

Making changes to invoices has no affect on invoices with non-inventory items on them.  Non-inventory items are expensed when purchased, so you need to change the bills used to purchase the items or create a journal entry that debits COGS and credits the GL account(s) that were used on the incorrectly-coded items.