Your agreement was based on your available time, quality standards, and other priorities, and, at the time, they agreed to the same thing. I wonder if their moving up the deadline is similar to someone who (reluctantly) agrees to a deadline they suspect they can't meet. I wonder if this person might have known they would want it earlier, but that they didn't have the guts to say so at the time.
Well, either that, or they are just inconsiderate, demanding, and difficult to work with. I still think restating the "contract" is the right path, with an appropriate reminder of the quality levels or conflicting priorities or whatever else that will likely suffer if you attempt to meet the NEW deadline. You planned your work around the agreement you made so you could deliver what they asked, when they asked. I guess in the spirit of being flexible and easier to work with, you could renegotiate any they are willing to give up in order to get this in their new timeframe (shorter? rough figures instead of tight ones?)
Ah, if we could just do our work without having to deal with the rest of it.