Known issue
There is a known bug in older versions of Oracle 11.2 where converting numeric attributes to string attributes with a length greater than 80 leads to corrupted data on existing rows.
This problem is fixed in Oracle 11.2.0.4. See https://support.oracle.com/epmos/faces/DocumentDisplay?id=9949330.8 and http://stackoverflow.com/questions/16735793/strange-behavior-on-oracle-cast-to-nvarchar2.