Ron Gilbert, formerly game designer at Lucasfilm Games and co-creator of The Secret of Monkey Island and Maniac Mansion, found and published some early design documents.
He writes: “If this document (and the Monkey Island Design Notes) say anything, it’s how much ideas change from initial concept to finished game. And that’s a good thing. Never be afraid to change your ideas. Refine and edit. If your finished game looks just like your initial idea, then you haven’t pushed and challenged yourself hard enough.”
Read the whole document at http://grumpygamer.com/maniac_mansion_design_doc