Upgrading problem

I'm a sort of new in the Linux world, so sorry for any boring, silly question I might post. Well, the present problem, among many others, is: I need to "export" my Red Hat 5.2's accounts/passwords (passwd file) to my new Red Hat 7.2, in a way that I do not have to create new passwords. I tried to copy an account line of 5.2's passwd file to the 7.2's, but it did not work. The problem is that the 7.2 does not accept the 5.2's MD5 passwords, or so I guess because no user whose 5.2's password is longer than 8 characters can log in. However, if I change these users' passwords for a new MD5 password (I mean a password longer than 8 ch.), it works fine. MD5 and Shadow are both enabled on the 7.2.