Yes I agree with the others - whilst both are great cameras they're really designed for different things.
The 5D (and 1Ds) are full frame high resolution cameras, the 1D is really optimised more for speed and telephoto type work, having a crop factor and superb frame rates and so on. So personally I'd only suggest the change if you were wanting to do faster action photography than a 5D is comfortable with, or if thats to become the main type of work you do, or if there were specific things with the 1D II that you felt you needed.
Just a thought, though: it may be a better upgrade path to save up for longer and then upgrate in a bigger leap to a more recent/current model. The original 5D is far from being a poor camera even though it has been superceded recently, so keeping it for longer may not be such a bad thing, unless its worn out or something like that, and the price of the 5D II seems to be falling from its introductory price at the moment (in the UK anyway).
Kevin