SQL & PL/SQL

I created this trigger, it works BUT I don't like those parentheses at the begining, I would like
to change those parentheses for case statements, well that is my question, can you use case statements in triggers, how you would translate the following in case statement?

FOR EACH ROW
WHEN ( (new.sgbstdn_levl_code = 'UG')
and
( (NEW. SGBSTDN_STST_CODE NOT IN ('GR','SA','AS','IS') )
OR
( (NEW. SGBSTDN_STST_CODE = 'IS' ) AND
(NEW. SGBSTDN_STYP_CODE IN ('N' , 'T' )) AND
(OLD. SGBSTDN_STST_CODE = 'AS' ) ) ) )

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CREATE OR REPLACE TRIGGER CC_STUD_WITHDRAWAL
AFTER UPDATE OR INSERT ON SATURN . SGBSTDN
FOR EACH ROW
WHEN ( (new.sgbstdn_levl_code = 'UG')
and
( (NEW. SGBSTDN_STST_CODE NOT IN ('GR','SA','AS','IS') )
OR
( (NEW. SGBSTDN_STST_CODE = 'IS' ) AND
(NEW. SGBSTDN_STYP_CODE IN ('N' , 'T' )) AND
(OLD. SGBSTDN_STST_CODE = 'AS' ) ) ) )
DECLARE
v_params gokparm.t_parameterlist;
event_code gtveqnm.gtveqnm_code%TYPE;
firstname spriden.spriden_first_name%TYPE;
lastname spriden.spriden_last_name%TYPE;
middlename spriden.spriden_mi%TYPE;
id spriden.spriden_id%TYPE;
CURSOR get_stud_name IS
SELECT
spriden_id ,
spriden_last_name ,
spriden_first_name ,
spriden_mi
FROM
saturn.spriden
WHERE spriden_pidm = :NEW.SGBSTDN_PIDM
AND spriden_change_ind IS NULL;
BEGIN
IF goksyst . f_isSystemLinkEnabled ( 'WORKFLOW' ) THEN
event_code := SUBSTR ( gokevnt.F_CheckEvent ( 'WORKFLOW' ,'CC_STUDENT_WITHDRAW' ),1,20);
OPEN get_stud_name ;
FETCH get_stud_name INTO id , lastname , firstname , middlename ;
CLOSE get_stud_name ;
----pass parameters to the event
v_params ( 1 ).param_value := 'CC_STUDENT_WITHDRAW' ;
v_params ( 2 ).param_value := '' ;
v_params ( 3 ).param_value := 'Student Withdrawal:' || lastname || ',' || firstname || ' ' ||
middlename ;
v_params ( 4 ).param_value := :NEW.sgbstdn_pidm ;
v_params ( 5 ).param_value := id ;
v_params ( 6 ).param_value := lastname ;
v_params ( 7 ).param_value := firstname ;
v_params ( 8 ).param_value := middlename ;
v_params ( 9 ).param_value := :NEW.sgbstdn_term_code_eff ;
v_params ( 10 ).param_value := :NEW.SGBSTDN_STST_CODE ;
v_params ( 11 ).param_value := :NEW.SGBSTDN_STYP_CODE ;
gokparm.Send_Param_List ( event_code , v_Params );
END IF;
END;
/