Just found this on MouseSavers, hope it works since I have some old tickets from WDW with days remaining!
Using Walt Disney World Tickets at Disneyland
Interesting, unpublished fact: you can use any unexpired, leftover days on a Walt Disney World ticket at Disneyland or Disney California Adventure! 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 Park Hopper ticket. You can repeat this day by day until the Disney World ticket is used up. They will not give you a multi-day ticket all at once, even if you have multiple days left on your Disney World ticket. Also, MouseSavers.com reader Danny P notes that any Disneyland ticket obtained from exchanging a Walt Disney World ticket must be used on the same day it was exchanged.
This policy has been in effect at least since I started this site (2001), and the information was last reconfirmed by a MouseSavers.com reader as of October 2010. Don't even bother calling Disneyland to ask about this -- the people answering the phone have no idea and they will tell you they don't take Disney World tickets.