SQL & PL/SQL

I need to update a column based on the conditions i used below, the update query i used is updating null values as well. How can i stop that and retain the old values when no match was found for the case.

create table sample (name varchar2(10),eno number(10),salary number(10));
insert into sample (name,eno,salary) values ('emp1',1,100);
insert into sample (name,eno,salary) values ('emp2',2,200);
insert into sample (name,eno,salary) values ('emp3',3,300);
select * from sample;
    
update sample 
set salary = 
case when salary = 100 then 10000 else 
case when salary = 150 then 15000 else 
case when salary = 200 then 20000 end end end
where name is not null;
       
Actual o/p:
       emp1	1	10000
       emp2	2	20000
       emp3	3	

Required o/p:
        emp1	1	10000
       emp2	2	20000
       emp3	3      300