Dave Mason goes against the flow:
One of the recurring questions I see on Stack Overflow is “How do I restore a SQL Server backup to a previous version of SQL Server?” The answer, of course, is you don’t. Upgrading a database to a newer (major) version is a one-way ticket–at least as far as the database files and subsequent backups go. I recently found myself in a similar position as all those hapless Stack Overflow questioners. I had a customer that had migrated to a newer version of SQL and they wanted to roll back to the previous version. What to do?
A couple of thoughts immediately came to mind. There’s the SQL Server Import and Export Wizard and the Generate and Publish Scripts Wizard. Neither of these sounded convenient. In particular, generating a script with both schema and 500 GB of data sounded like a fruitless endeavor. Two other options sounded much more appealing. So I focused on those.
Dave has a couple of creative methods effectively to downgrade a database.