Function of DROP CAST
DROP CAST drops the cast that converts the specified source data type to the specified target data type. Either the source data type or the target data type or both must be a UDT.
The system does not drop the associated external cast routine when you perform DROP CAST.
There must be an existing cast definition to be able to drop that definition. If there is no existing cast operation defined for the specified conversion, the request aborts and the system returns an error to the requestor.
You cannot drop a cast if it is referenced by a view, stored procedure, trigger, or macro.