SQL 2012: Import DB changes original data scheme
Take a backup of the SQL Server 2008 databases and restore it on SQL Server 2012. It is a much faster and reliable approach to migrating a database. Do not attach files between the instances, the SQL Server 2012 will upgrade them to a new format and you will loose the ability to use them again on SQL Server 2008. 

As a general advice try to follow the procedures and guidelines, including the pre-migration validation steps from Upgrade to SQL Server 2012

Make sure you migrate all login information and SQL Agent jobs relevant for your database(s).