A new value is not to be assigned to the field symbol

Hi,

i defined field symbol  like this:

   FIELD-SYMBOLS:   <LFS_01> .

i changed it_demand-fsc_seq field type from num2 to num3..i have below loop..

LOOP AT it_demand.

     MOVE-CORRESPONDING it_demand TO IT_OUT.

     CONCATENATE  'IT_OUT-DMD'  it_demand-fsc_seq INTO L_FIELD_NAME.

     ASSIGN  (L_FIELD_NAME)  TO <LFS_01>.

     <LFS_01> = it_demand-demand.-------> on here.. an error accured.

     COLLECT IT_OUT.

     CLEAR IT_OUT.

   ENDLOOP.

Error message...ST22

A new value is to be assigned to the field "<LFS_01>", although this field is

entirely or partly protected against changes.

The following are protected from change:

- Character literals or numerical literals.

- CONSTANTS.

- Parameters of the category IMPORTING REFERENCE in functions and

   methods.

Before i changed it_demand-fsc_seq field type, there was no error..How can i fix that ?