I too have the same scenario where in the column was decimal and updated to bigint, now getting error while querying the column. the data type on the table and the parquet file are aligned.
Error: java.io.IOException: java.lang.ClassCastException: org.apache.hadoop.io.LongWritable cannot be cast to org.apache.hadoop.hive.serde2.io.HiveDecimalWritable (state=,code=0)
If you have already resolved the issue, much appreciate if you could let know what worked for you.
Thanks
Dinesh