Change field via validation Exit

Hello,

I am trying to update a filed (BKPF-BLDAT) thru a Validation Exit, but it's not working.

here is my code

FORM Z002 USING bool_data TYPE gb002_015
           CHANGING B_RESULT.

     IF SY-SUBRC = 0
       BKPF-BLDAT   = SY-DATUM

       B_RESULT      = B_false.
    ENDIF.

ENDFORM.

when debbuging, the BKPF-BLDAT is well updated, but value is re-initiated further on after exit end.

is that possible to update the field as I am doing or it's not how I should operate ?

PS : I used a substitution to solve my issue but I am interested to know if that was possible thru validation exit.

Thank you