Handling lossy data before character- set migration
Straight from the Manual:


  The easiest approach to migrating convertible data dictionary data is to create a new database in the target character set and then re-create all of the data dictionary and schema definitions by exporting and importing. However, this method means creating a new database.


Based on what I'm reading in this manual, that's hands-down the safest and easiest way to do this migration.