The password columns will need to hold 40 characters. If you have updated the table for this then any new users will be able to register and login without issue. However the users who had already registered and had only the first 20 characters of their password stored in the db will not be able to access the site.
This is because when they login and enter their password the entire password will be hashed and compared against what is in the db for them. Since the existing stored password for these users will only have 20 characters the password they enter will not match. To remedy this problem you will need to set the password again for these users. You could get the password from the user and get a hash for it to update the db with but this is more involved. The best thing to do would be to send those users an email and let them know that they need to return to the site and register again. You will not be able to have them just update their profile because they will not be able to login.
