SQLRNNR (3/30/2015)
The documentation poses an interesting conundrum for which I previously submitted connect items (MS closed saying they won't fix).If you try to change to compat 90 (sql 2005) you will get this in a 2014 instance.
Msg 15048, Level 16, State 3, Line 3
Valid values of the database compatibility level are 100, 110, or 120.
Msg 5069, Level 16, State 1, Line 3
ALTER DATABASE statement failed.
Since the documentation says that you can upgrade from 2005, I will pull up another test and confirm it.
When you restore a 2005 DB to a 2014 instance it will default your compatibility level to 2008(100). I do this with our DB backups every 3 days to test them. Hopefully that adds some info so you don't have to go around practice restoring 🙂