HashJoin currently does not allow schema change in either build side or probe side. When HashJoin hit SchemaChangeException in the middle of execution, Drill reports a brief error message about SchemaChangeException, without providing any information what schemas are in the incoming batches. That makes hard to analyze the error, and understand what's going on.
It probably makes sense to put the two differing schemas in the error message, so that user could get better idea about the schema change.
Before Drill can provide support for schema change in HashJoin, the detailed error message would help user debug error.