muy buen punto... a mi tambien me puso a pensar...
anexo ejemplo...
Código:
If object_id('tempdb..#tmpUser') is not null Drop Table #tmpUser
Create table #tmpUser (idUser int identity (1,1), ClaveUsuario varchar(10))
Insert into #tmpUser values('uno')
Insert into #tmpUser values('dos')
Insert into #tmpUser values('tres')
Insert into #tmpUser values('cuatro')
Insert into #tmpUser values('cinco')
Insert into #tmpUser values('seis')
Select * from #tmpUser
update #tmpUser
set ClaveUsuario = Left(Cast(newid() As VarChar(50)), 8)
Select * from #tmpUser