Is it possible to change all my invoices under an account to reflect one business location? We are having to manually select the business for each invoice (yikes).

Yes, you can edit it manually through your invoice, @ryan56.
 
I’d be glad to show you the steps on how to edit the sales location in your invoice below.
 
Here’s how:
 
Log in your credentials in QuickBooks Online.
Go to the Hamburger icon.
On the left pane, select the Sales tab.
Click on the Invoices tab then select the drop-down icon and choose View/Edit tab.
Select the box for Location of sale and modify.
Hit Save.

I also Include this screenshots for your reference with the step 4.

Also, for future reference, saw this to be handy for you: Tips for year-end reconciliation.

Please comment below if you need further assistance with modifying the sales location of your invoice. I’ll be around and ready to give you a helping hand. Take care.

Thank you. We knew of this at the time of the post and we’re looking for a way to batch change them. 

thanks!

Thanks but we are looking for a batch solution. We have a lot of invoices. 

You're unable to edit the invoices by batch, Ryan56.

You'll want to change the invoice one by one manually.

I know it's convenient editing the invoices by batch. I'll surely send your concern to our management team. They'll help us submit this to our product engineers. This will be reviewed and might be added in future updates.

Please know we're just around if you have other concerns.