You can no longer turn in partially used MYW tickets to get a dollar credit that can be applied towards the purchase of a longer MYW ticket or an AP/PAP.
You can "upgrade" to a longer ticket or AP/PAP, but only if you do it within 14 days of the first use of the original MYW ticket.
FWIW, getting a dollar credit is not in your best economic interest anyway. MYW tickets are very heavily front end loaded - the last days of a 10 day ticket can be worth as little as $9 or $10 each. Much better to save the remaining days and use them for admission.