Here's some ticket info from mousesavers.com
Unlike tickets and passes at Walt Disney World, all multi-day passes at Disneyland expire 13 days after the first day you start using them. (Unless you use them up first. Naturally a 3-Day Park Hopper is "expired" once you've used all three days, even if 13 days haven't passed yet!)
Another important thing to know: you can apply the value of an unused ticket or pass, or even a partially used one with days left on it, toward the purchase of a higher-priced pass, as long as you do it before your pass expires. For instance, if you buy a 3-Day Park Hopper and then decide on the third day of use that you'd rather have an Annual Pass, you can apply the value of the Park Hopper toward the Annual Pass. Discounted passes will be credited at the actual rates you paid. Yes, Disney can tell how much you paid. It's encoded on the pass. To upgrade a pass, go to any Guest Services window.
If you buy a restricted Annual Pass (such as the Southern California Annual Passport, which has many "blackout dates"), but you plan to start your trip on one of the blackout dates, the blackout will not apply on the day you purchase the pass. So if you go to Disneyland on a Saturday and buy an Annual Pass that is normally not valid on Saturdays, you will still be able to use it that day at both parks.
Old Disneyland passes that were issued in the days before expiration dates applied can still be used. Their value is too low to be worth applying to a new pass, though. Just use them for admission (Guest Services will trade them for passes that will work at the gates) -- or if they're old enough, they might be worth more as collectibles!
Interesting, unpublished fact: you can use any unexpired, leftover days on a Walt Disney World ticket at Disneyland or DCA! Present the Disney World ticket at the gate on arrival -- they will take it somewhere, deduct a day from it, and give you a one-day Disneyland and/or DCA ticket. The type of Disneyland ticket you will receive is based on the Walt Disney World ticket you present: if you have a Disney World ticket with the Hopper option, you will get a Disneyland Hopper that allows you to visit both parks in the same day; on the other hand, if the Disney World ticket had no Hopper option, you will get a one-day/one-park ticket and will have to pick which park to enter that day. This information was confirmed as of May 2006.
Disneyland passes are NOT accepted at Walt Disney World, and Annual Passes are only good at the park where they were purchased (WDW Annual Passes are NOT good at Disneyland, and vice versa).