SQL & PL/SQL

Dear all;

I have the following query and table below

create table t1
( 
       cid varchar2(200),
       yearid varchar2(200),
       begin_date date
       );

 insert into t1
           (cid, yearid, begin_date)
         values
           ('A', '2010_1', sysdate);
  insert into t1
           (cid, yearid, begin_date)
         values
           ('B', '2010_1', to_date('03/04/2011', 'MM/DD/YYYY'));
insert into t1
           (cid, yearid, begin_date)
         values
           ('C', '2010_1', to_date('01/02/2011', 'MM/DD/YYYY'));

and I have this update statement below

   update t1 t
       set t.yearid = '2011_1'
       where to_char(t.begin_date, 'YYYY') = substr(t.yearid, 1, 4);

what i am trying to is basically update table t1 yearid which is a varchar2 where it has to_char begin date that are irregular...so in my sample case,

to_char of the begin date return a 2011
hence, the yearid should be 2011_1 instead.

However, my udate statement isnt working though