I'm being told the change is included in my scope

It is the standard defence to a variation claim: the drawings were always going to develop, the specification said suitable for purpose, and your tender should have allowed for it. Between genuine design development, which you do carry, and a change to the works, which you do not, sits a line, and the other side has every incentive to draw it in their favour.

The line is found in the documents you tendered on: what was shown, what was reasonably inferable, and what the contract says about design responsibility. That analysis decides each item one way or the other, and turns a shouting match about fairness into a document exercise you can actually win.