How do you plan a series?

My recently finished WIP 2, is supposed to be a Fantasy Trilogy. I know what happens at the very end, and I know some things that happen in between, but that was about it when I wrote the first draft. After that first draft though, I have decided to replot. I am not changing the essence of the story, but I am making significant changes in the story line itself, and cutting off some characters, so this time I was wondering if I should have a bit more detailed plan for the whole trilogy before starting the re-write of the first book? How do you plan trilogies or duologies etc? How much do you know about the next book in the series? Things could change of course when you come to the second book, but I feel that it might make the first book better to know bit more details about the next two. Perhaps that might help adding more layers to the story. What do you think?