How do you change the a/r account in an invoice? For example, I have several different a/r accounts depending on the type of invoice.

I can provide you the two methods in changing the A/R account in an invoice, @kdevilliers. 

Maintaining a single A/R account is a general recommendation in organizing your books in QuickBooks Online (QBO). Since you want to set up invoices for your several different A/R accounts, there are two options you can perform to achieve your goal. 

• Set up parent and sub-customers to group your open receivables
• Use location tracking to group receivables

For the first method, let's create a parent customer for every A/R account you have. This way, you'll be able to assign an account to your customer. I'll guide you how. 

1. Go to Sales from the left menu. 
2. Select Customers.
3. Click New customer to open the Customer information dialog.
4. Enter the A/R account name in the Company field.
5. Hit Save.
6. Repeat the process for the other A/R accounts. 

Then, let's determine which customers are sub-customers of the parent one you created above. Please make sure to use the customers that are associated with your existing invoices. Just go back to the Customers tab. Then, proceed with the steps below. 

1. Locate and select the existing customer to change.
2. Select Edit.
3. Check the Is sub-customer box.
4. Choose the parent customer from the Parent drop-down list.
5. Hit Save.
6. Repeat the process for the other A/R accounts. 

The screenshot below shows you the third to fifth steps. 

Once done, the system will automatically update the invoice using the specific A/R account in the Customer drop-down. To check it, locate the invoice from the Invoices tab. Then, review the Customer/Project column. View the screenshot below for your visual reference. 

For the second option, I'd suggest using the location tracking feature in the program. That way, you'll be able to group your receivables by location, depending on the type of invoice. For more details, see the Use location tracking to group payables or receivables section through this article: How To Group Accounts Receivable or Accounts Payable Account Types. 

I recommend running the Customer Balance Detail report from the Reports menu. It helps you ensure that your invoices and other transactions are associated with an A/R account. I've attached a screenshot below on how the report looks like in QBO. 

I'm just a comment away if you need anything else. I'll gladly help, @kdevilliers.