From allearsnet.com
DOLLAR CREDIT
Disney allows you to apply the DOLLAR CREDIT of any unused major park admissions left on your ticket towards the purchase of a longer MYW ticket/AP/PAP. There is no time limit on this as long as you have the no expiration option on the original ticket. Unused plus options carry no dollar value when figuring DOLLAR CREDIT.
MYW tickets are front loaded in value. That means that the most expensive days are the earliest ones. The later days (example: 8, 9 & 10 on a 10 day ticket) are only worth less than $5 each. The basic formula for the amount of the credit will be a deduction of a one day ticket price for the first day you used, the amount of the first and second day for the first two days you used, etc. By the time you get up to day 5, your ticket is actually worth very little in value towards a new one.
The ticket will be credited with the gate price amount, not just the puchase price amount for the purposes of doing the upgrade. When you upgrade these tickets, you will only pay the price difference between the value of the remaining park admissions on the old ticket and the price of the new ticket. You will not be charged for any other price difference. Important note: Problems seem to occur with this often when tickets purchased in advance from a non-Disney source are involved. Guest Relations is much more consistent in doing this transaction correctly than a resort's Lobby Concierge is.
You will only be allowed to apply one old ticket to each new one you get. You cannot take five old tickets each with a day on them and apply them all to one new MYW ticket/AP/PAP. If you were getting 5 new tickets then you could use all five of the old ones by applying one to each.
If the old ticket was used in more than one park on any one day, you will be required to purchase the new ticket with the Park Hopper add on. If the old ticket was used at a water park, PI, WWoS, or DQ, you will be required to purchase the new ticket with the Water Park Fun & More add on. If it has been longer than 14 days since first use and you had the no expiration option on it, you will be required to purchase the new ticket with the no expiration option. If the old ticket was unused, then you will be allowed to purchase any MYW ticket you choose so long as the cost of the new ticket exceeds that of the old one.
http://www.allearsnet.com/pl/ticket.htm#credpass