You can only restore them to the same version they were backed up from. As stated in BOL:
System databases can be restored only from backups that are created on the version of SQL Server that the server instance is currently running. For example, to restore a system database on a server instance that is running on SQL Server 2005 SP1, you must use a database backup that was created after the server instance was upgraded to SQL Server 2005 SP1.
Shawn Melton
Twitter: @wsmelton
Blog: wsmelton.github.com
Github: wsmelton