Looking at your log, it seems like you are trying to change the datatype in Spark. Is this the case? If yes, use the statement like
val a = sqlContext.sql("alter table tableName change col col bigint")
Talking about the issue you are facing while converting the type of the column, you need to understand the available datatypes and the implicit cast option available between them.
So whenever you issue a command like
alter table tableName change columnName columnName <newDataType>;
You need to understand that you may have some data in your Hive table's column which is string type now and if you are casting to a variable with datatype like int etc, you may not be able to access certain values and they will generate null.