"Cannot issue ALTER DATABASE NATIONAL CHARACTER SET when NCLOB, NCHAR or NVARCHAR2 data exists"
*Cause: NCLOB, NCHAR or NVARCHAR2 data changed the representation to Unicode
when converting to a multibyte character set and must be migrated.
*Action: Remove NCLOB, NCHAR or NVARCHAR2 data as listed in the alert file.
The above type data can be migrated by methods such as import/export.