Hive Data type conversation issue on partition table

[#1]

We had a table, partition on date where table had column as varchar, 2 months before table has been changed and the column change to int.
Now for the specific date when trying to fetch data, its giving an error.
Kindly guide, how do i come out if this situation.
Please note data are fine through out table and have problem for some specific date.
where if i do select * from table between the date is giving me the output, but when i am trying  to do count based on the changed column data type is throws below error.

Error: java.io.IOException: java.io.IOException: java.lang.ClassCastException: org.apache.hadoop.io.IntWritable cannot be cast to org.apache.hadoop.hive.serde2.io.HiveVarcharWritable

[#2]

I changed the table syntax from
STORED AS INPUTFORMAT
'org.apache.hadoop.hive.ql.io.orc.OrcInputFormat'

to
STORED AS ORC;
fixed the issue of count(*). but looking for explanation, if any one can help me out.

[#3]

This is because some files we written with old schema and some files were written with the new schema. Depending how you query the data (if it is a select count(*) or select column) it may or may not fail. I dont know too much about ORC files, but in parquet files you can download the files to a local machine and via parquet tools examine the schema of each binary.

If you are able to read the whole table, I suggest to do 

insert overwrite mytable select * from mytable;

Just to make sure, that every file is created with a correct schema