We had the same question, so I e-mailed Disney through the Disney World Website & here is their response:
Dear Cathy,
Thank you for contacting the Walt Disney World® Resort.
Unused days on admission tickets (which have no printed expiration date) may be
used for admission to our theme parks on any future visit; however, when
children become older than the established "child" age range, they will require
the adult priced admission media to enter our theme parks.
In these circumstances, existing passes may be presented in person at any Guest
Relations location, to have the tickets exchanged for the required media (at no
additional charge). As another option, the remaining value of the tickets may
be credited toward the purchase of a new multi-day ticket.
All ticket upgrades must be done in person. Please present your tickets at any
theme park ticket window or Guest Relations for assistance. Some variables may
determine if an upgrade is possible.
For up to date information on ticket media that is available for the WALT DISNEY
WORLD Resort, please visit our Online Ticket Store at the following URL:
http://attractiontickets.tickets.com/buy/TicketTrans
For any other questions regarding theme park tickets, please call 407/560 PASS
(7277).
Hope this helps.
