The pam_pwquality module's 'difok' parameter controls requirements for usage of different characters during a password change. Modify the 'difok' setting in '/etc/security/pwquality.conf' to require differing characters when changing passwords. The DoD requirement is '4'.

[4_diff_char]

Requiring a minimum number of different characters during password changes ensures that newly changed passwords should not resemble previously compromised ones. Note that passwords which are changed on compromised systems will still be compromised, however.