How can I change the shipping account for past invoices?

Thanks for visiting the Community, @JayBlake.

The option to change previous shipping accounts on previous transactions in QuickBooks Online (QBO) is unavailable. 

You can create a journal entry instead to transfer the balance to the correct account. I also recommend seeking assistance from your accountant to ensure that your books are accurate and to avoid messing up the data. Otherwise, you'll have to manually delete and recreate each of your transactions.

Additionally, you can focus on the details that matter the most to you when you pull up reports in QBO. If you wish to know more about this process, I'd recommend checking out this article: Customize reports in QuickBooks Online.

Let me know in the comments if you have other concerns about managing your sales transactions in QBO. I'll gladly help. Take care!