Hi Rachid,
As Suhas Saha was explaining you seem to have a simple cast error. In the following code obj1 class is a subclass of obj2. Casting like this won't be a problem. The precise inverse will:
DATA: lo_obj1 TYPE REF TO lcl_sub_class.
DATA: lo_obj2 TYPE REF TO lcl_parent_class.
FIELD-SYMBOLS <ls_1> TYPE any.
CREATE OBJECT lo_obj1.
ASSIGN lo_obj1 to <ls_1>.
lo_obj2 ?= <ls_1>.
regards,
Edgar