Although you may not be able to use a WDW annual pass at DL, You definitely can use the left over days from your WDW park hopper or even the old value pass which isn't sold anymore.
I've used the left over days on my 1996 value pass from WDW at DL as recently as last year. I called to double check if I could also use it at California Adventure and was told, yes, I could. Take your pass directly to the gate. The first time I did it, I had to wait a while because they said they weren't hooked up to WDW to figure out how many days I had left on the pass. THey had to phone them. Last year when I used it, it went right through so they must have solved that problem.