SQL Server 2016 CTP -> RTM, safe to attach CTP DB files (including SSISDB)?
You can safely attach any database from a prior build. As soon as you do this, the internal database structure will be automatically upgraded if there are any changes. This includes system databases, but they will not be considered system databases, using this method.

You can still retrieve SQL Server Agent jobs from the old msdb database if you need them, as well as anything you wanted from master, including things like logins.