hi there
i might have to disagree on this, if you have quoted for an amount of work, and the parameters ave changed then you do have a commercial argument.
BUT, you need to guage this with the client, to decide if you have the relationship with them that will allow you to obtain an uplift to the price.
i have recently done that, purely on the fact that the goal posts have been moved, but it was done in a professional and commercial manner.
good luck
regards
martin