In Oracle Database 12c, the RMAN “DUPLICATE” command has new “NOOPEN” option which clones the database but does not open the auxiliary instance. This feature can be used to duplicate a database to higher version.
MOS Note: 2022820.1
Upgrade to 12c database using RMAN Duplicate with NOOPEN clause
The note contains also a full example on how to duplicate the production database as a standby, and upgrade it.