You have to be careful here...
current policy taken in part from allears
http://www.allearsnet.com/pl/ticket.htm#credpass
Can I upgrade my ___ (fill in the blank with a ticket name) ticket to a longer ticket or an Annual Pass?"
Yes you can as long as it is a Magic Your Way ticket and you do the upgrade within 14 days of first use or a
completely unused older ticket.
If you have any partially used Disney World admission tickets that were purchased prior to the January 2005 introduction of the Magic Your Way tickets, the only thing that you can do with them now is use up the remaining admissions at the parks. Those tickets can no longer be used for upgrading or for applying their dollar value towards new ones.
If you are upgrading a pre-2005 ticket that is completely unused, it must be done at Guest Relations Resorts cannot do any upgrades that involve Annual, Premium Annual or Seasonal Passes.
Disney has been quietly changing the way they handle ticket changes during 2006. The most noticeable changes are a move away from giving dollar credit on previously used tickets and the guidelines as to how upgrades are handled.
Disney allows you to
UPGRADE most park tickets to a longer Magic Your Way ticket, an Annual Pass (AP), a Premium Annual Pass (PAP) or a Season Pass for Florida Residents (SP) within the first 14 days for MYW tickets that do not have the no expiration option on them. They will take the entire price of the ticket you started using and deduct it from the cost of the new ticket.
Disney will not allow you to upgrade certain promotional tickets not available to the general public or any partially used ticket purchased prior to the January 2005 introduction of Magic Your Way tickets.
You can upgrade your MYW ticket to an AP/PAP/SP. You will receive full purchase price on the original ticket and you will pay only the difference in price to the AP/PAP/SP. The starting date of an AP/PAP will be back dated to the date you first used any admission off the old ticket. This transaction must be done within 14 days of first use if the ticket is an expiring one.