[logfromblammo]

The password-change form should be using a password field, and that should not be allowing any code or scripts to grab the plaintext stored in it.

If the code that compares your current password to the new password can read the plaintext of your passwords, so too could a malicious program.

Using HTML input type="password" alone is not sufficient protection. The same steps that protect password changes from malicious attackers must necessarily protect them enforcement of bad IT security policy.

[Sohcahtoa82]

The check is done server-side.

At the time of a password change, the server still has your old password hash stored, and in the process of changing it, you are sending both your old password and new password. The server can verify both that your new password and old password differ enough while also verifying that the old password you sent it is valid.