If you have forgotten the old password, then you cannot change it. It is that simple.
Follow the logic here. I gain access to a computer that you logged into at the library. You being a novice internet user, never cleared the cache, and left the checkbox that says, "keep my logged innto this computer" checked. I then use the same workstation, and I open the history. I them proceed to gain access to your already logged into account, and attempt to change the password. IF it did not challenge me for the old password, then I wold have just gained access and control over your account. The fact that it does challenge me for the old password, I cannot enter it, and you still have control over the account, even though I can post all sorts of cool stuff on your already logged into account! see how it works?