Here is a post off the Disboards that discusses this. The person who posted used to work in ticketing;
Almost all tickets issued prior to 2005 (other than Length of Stay, Bounce Back, or Annual Passes) had no expiration. This includes both the major park entries and the minor park plus options.
At the current time any prior passes are valid at all four of the major parks, even if one or more of the parks did not exist when the ticket was issued.
If a prior ticket, unused or partially used, was not on stock with a magnetic stripe, or the magnetic stripe was demagnetized, you can get the tickets replaced at no charge at any park (including water parks or TTC) ticket booth or Guest Relations Office. Note that this cannot be done at Resort Lobby Concierge locations as they do not have full access to the ticketing computer.
No pre-MYW ticket can be modified or upgraded in any manner.
If you have unused pre-MYW tickets you can use the original Disney selling price of one ticket as a credit to the purchase price of any current ticket with a selling price equal to or higher than the value of the old ticket. However, it probably will make more economic sense to use the ticket for entries. This will depend a lot on what the determined dollar value is.
Note that if the ticket was purchased from a discounter/broker the value given is what Disney sold the ticket for to the discounter/broker and not what you paid for it.
If you have a partially used pre-MYW ticket it cannot be upgraded, but the remaining entitlements can be used. If the ticket has become demagnetized, as long as the serial number string is readable a replacement ticket, with the same remaining entitlements, can be issued.