A demagnetized ticket, whether MYW or pre-MYW, can be replaced at any park (including water park) ticket window, Guest Relations location, or even at the gates themselves. Each of the parks has a location inside at the turnstiles so if your ticket is demagnetized the greeter will ask you to wait a minute and will take it to the person who handles this and you will get a new ticket immediately.
If you buy a MYW ticket the "clock" for expiration does not start until the ticket has gone through its first turnstile, so you can keep it for years and it will retain its value. Actually it may increase in value as
ticket prices go up.
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.
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 partically used pre-MYW ticket it cannot be upgraded, but the remaining entitlements can be used. If the ticket has become demagetized, as long as the serial number string is readable a replacement ticket, with the same remaining entitlements, can be issued.