Thanks Pavel and Woody. I fixed my data by doing this:

step 1:
update clients
set firstname = 'Unknown'
where firstname is null

step 2:
update clients
set lastname = 'Unknown'
where lastname is null

step 3:
update clients
set middlename = ''
where middlename is null

step 4:
update clients c1
set c1.client_id =
(select min(c2.client_id) from clients c2
where (c2.firstname = c1.firstname) and
(c2.middlename = c1.middlename) and
(c2.lastname = c1.lastname))

Clay Shannon,
Dimension 4 Software