Here's the REAL SOLUTION
This is another of Intuit's famous changes to the system that they regularly do without consulting or informing the users (who pay a very dear price for this platform).
Once the estimate has changed the status to "Converted", the "Update Status" choice in the "Actions" drop down box is gone.
You must Edit the estimate (from the "Actions" column) and click on the "Manage" gear icon. Then your first drop down box (it will say "Converted") will allow you to change the status to "Accepted". Save the estimate (you don't need to send it again). Close the estimate. Now the "Actions" column will contain "Convert to invoice" link. Click on that and create your invoice as normal.
Thanks Intuit for another feature we didn't need or ask for.
