What if our idea changes mid build?

Normal and expected. Most startup MVPs we have shipped looked different at launch than at scoping. We structure engagements with weekly check ins, two week iteration cycles, and a clear scope change process: you can swap a feature out for one of equal complexity at any sprint boundary without renegotiating contract. Bigger scope changes (significant timeline or budget impact) get a written addendum but do not require new contracts. We have ridden out plenty of pivots, including a full repositioning at week six of an MVP build. It works as long as the change conversation happens openly and early.