Trigger problem

Hi there, I'm having a problem, When I'm doing an update to one of the
fields of my table I need to know if it's changed.
For exemple I have the table with the columns
id,identity,registration_state.

Test:
id | identity | registration_state
1 | Joe | registered
2 | Tom | not_registered
3 | James | unregistered

when i do "update test set registration_state='registered'" I need to know
for which identities the registration_state was change (and then return it).
In this case, Tom and James.

I was thinking about using a trigger function for the updates, using
pl/pgsql (using before in the trigger). the problem is that I don't know how
to get the old registration_state when I have multiple rows affected. I
known that if I use OLD.registration_state it works for one rows. Can you
help for multiple? tks a lot in advance