To upgrade PDBs after upgrading the Container Database (CDB), use the unplug-plug technique.

• Unplugging the Earlier Release PDB from the Earlier Release CDB
  To prepare for upgrading the PDB, use this procedure to unplug the PDB from the earlier release CDB.
• Plugging in the Earlier Release PDB to the Later Release CDB
  To Plug the PDB from the earlier release CDB to the later release CDB, use the CREATE PLUGGABLE DATABASE command.
• Upgrading the Earlier Release PDB to the Later Release
  Open PDBs in UPGRADE mode use the Parallel Upgrade Utility to carry out the upgrade of the earlier-release PDB to the release level of the CDB.
• Use Inclusion or Exclusion Lists for PDB Upgrades
  If you want to upgrade a subset of earlier release PDBs, then use inclusion or exclusion lists to avoid reupgrading the CDB or PDBs that are at the new release level.