SQL & PL/SQL

i have in tb1 data like and i need the latest status changed date : when date on which the status of the task changes.

for the data below for taskno :1 i need to get the o/p : 1/2/2013

and for taskno :2  i need the o/p 1/4/2013

im writing the query as below

select taskno, max(upd_date) MAX_UPD_DATE from (

select taskno, upd_date, old_status, new_status, rank from (

select taskno, upd_date, old_status, new_status, rank() over (partition by taskno, old_status, new_status order by upd_date asc) as rank

from tb1

)

where rank = 1

and old_status <> new_status)

group by taskno;

IS there any more efficient way to write this query as the clause "old_status <> new_status" is taking a toll on the performance