Thanks for the quick reply, @bweiss.

I'll share some knowledge about sub customers in QuickBooks Online. 

The reason for that is to ensure the customer's transactions will not be messed up. 

To your second question, Yes, there's a way to use the same number with its existing invoice. You can edit the number while recreating them since the deleted invoice will not show up in the customer's transaction list. 

See the sample screenshot below. 

Let's try merging the customer as another work around.  This deletes one profile and moves their existing data into the profile you want to keep.
 

1. Go to the Sales tab, then choose Customer. 
2. Hit the New Customer button.
3. Type same information with sub-customer, then tap Save. Make sure the new customer doesn't have any transactions. 
4. Click the sub-customer, then remove the checkmark on Is sub-customer box. 
5. Hit Save, then Yes to merge them.
6. Re-click the new customer, then add the new parent.

For more details, check out this article: Add and manage customers in QuickBooks Online.

I've also added an article about handling transactions, customize sales, forms, and other topics: Sales and customers.

Let me know how the workaround goes. I'm happy to assist further.