WhiteWebServices - 11:24 am on Jul 12, 2007 (gmt 0)

Not from a legal perspective, but:

I think it would be fine for you to ask to renegotiate your rates after a reasonable period of time has passed. However, mid-way through the first project on the new terms is too soon; as Andye said, even if you didn't sign a contract, you did have an agreement.

Really it would be best if you were in a position to say, "Since the original agreement x, y, and z have changed" in order to justify renegotiating terms. The longer you leave it, the more plausible it is that enough has changed that the rates originally agreed should no longer apply.