Unused tickets do "expire" in that they cannot be used as-is. However you are correct that they will always retain their value as a credit towards new dated tickets. Even if you have to wait a couple of years (or more). Just make note of the ticket numbers because they are likely to disappear from your view, though WDW can still see them. It would be good to note the Disney online value at the time you purchase as well, because that is the amount of credit you should have (and that info may be hard to find later).