I had the same question as we have 10 day non expiring hoppers with 7 days remaining. I am only going to DL for one day (day-tripping from San Diego) so a one-day hopper for me would be $83. Not a bad exchange for one day off of my WDW MYW hopper. In any case, here are the official responses from the
Disneyland Guest Communications - 1st response was whether or not the tickets could be exchanged and the second response is regarding the hopper option. Hope this helps.
#1
WALT DISNEY WORLD® Resort single and multi-day tickets with remaining
uses should be accepted at the DISNEYLAND® Resort. Please note that
complimentary tickets or Annual Passes to the WALT DISNEY WORLD® Resort
are not valid for admission. Regrettably, we are unable to guarantee the
validity of tickets in advance by e-mail or telephone, as our Cast
Members must physically see the tickets in question. Please present the
tickets to a Cast Member at any Main Entrance turnstile for
verification.
#2
You will be pleased to know that if the Walt DisneyWorld ticket is a
hopper ticket, it may be used at Disneyland as a hopper ticket.
Megan