When you find yourself needing to perform a character set migration as part of a database upgrade or migration, the Oracle Database Migration Assistant for Unicode (DMU) is a very helpful tool. It will assess your migration needs and help automate the process, all with a very nice GUI front end that makes the whole process easier.
And now comes a really nice enhancement with DMU 2.1: automation of near-zero downtime character set migration! Here is the blurb from the DMU OTN page:
New! Oracle DMU 2.1, released in May 2015, supports a near-zero downtime migration model in conjunction with the Oracle GoldenGate replication technology. Using DMU 2.1 and GoldenGate 22.214.171.124.0 or later, you can set up a migration procedure that takes advantage of the DMU data preparation and in-place conversion capabilities while leveraging GoldenGate to replicate incremental data changes on the production system during the migration process, thereby effectively eliminating the downtime window requirement. Other new features in DMU 2.1 include migration profile support, problem data report, and transparent repository upgrade. Please see the DMU 2.1 Release Notes for changes since the 2.0 release.
See the OTN Page for DMU to get all the latest information.