Check out sp_change_users_login in BOL. Specifically the second example at the bottem.
This example changes the link between user Mary in the pubs database and the existing login, to the new login NewMary (added with sp_addlogin).
--Add the new login.
USE master
go
EXEC sp_addlogin 'NewMary'
go
--Change the user account to link with the 'NewMary' login.
USE pubs
go
EXEC sp_change_users_login 'Update_One', 'Mary', 'NewMary'
I know it works in 2k, but not sure if it was in 7. fairly sure it was though.