Another NAV2017 cumulative update 12 AU issue

I had to completely uninstall and reinstall the NAV server and client so I could run the Demo database, and everything is updated.

But I cannot then access the company's database. I cannot run it from the NAV Server: I get an error (seen in event viewer) that it must be converted. In the development environment NAV2017CU12, running as administrator, I open the company database, am prompted to convert, I click on yes, then I get an error that I do not have permission to delete from the G/L Entry table. The service account and my account are both db_owners of the company database.

Why is it deleting from the G/L Entry table anyway?

So I cannot run the database until I convert it, and I cannot convert it. Also I get an error when I attempt to import the new version of table objects, that it cannot find the instance, so I cannot import the objects.

Slawek warned me that I would not be able to access my old database with the new client. I still have the NAV2017 software on the drive, so I can access the database with the development environment using that finsql.exe. But it doesn't really help. I cannot find in the upgrade codeunit where it even uses the G/L Entry table.

Thanks for any suggestions.