Here is a C&P from allears.com
Disney allows UPGRADING only within the first 7 days of using the ticket for PH/PHP tickets, or the first 14 days for MYW tickets. After the seventh/fourteenth day, they will not upgrade it but they will apply the DOLLAR CREDIT of any unused major park admissions towards the purchase price of the new ticket.
UPGRADING is the act of applying the entire original purchase price towards the new MYW ticket or AP/PAP. DOLLAR CREDIT is the act of applying the current dollar value (not the entire purchase price) of the unused major park admissions on the ticket towards the new one. Unused plus options carry no dollar value.