Your solution worked for me *until* I deleted the outstanding "Test Customer" invoice. It then reverted to the old numbering. Perhaps if I'd sent an invoice after the Test Customer's invoice (and before deleting the test one), it might have kept the new numbering. 
It's a decent amount of effort just to get the invoice numbering altered at all, and to then revert is just so very frustrating. I cleared the browser cache (relogged in to all the sites I access with this browser, including QBO), then followed your directions. When I opened up blank invoices they were showing the new numbering. But I didn't have one to send right away and I didn't want the test invoice messing up things...I should have waited.  
So fair warning to others: Leave the test invoice alone until you've had other invoices with the new numbering. 

Why QBO has not provided a more reasonable fix for this is beyond me.