A client needs notice that changes or expansion of scope could result in additional charges; if they had such notice, then you can charge them. So if either your original written agreement with them warned them that changes or expansions would result in charges, or you notified them when they requested extra work that you'd have to charge them more for doing so and they ratified or agreed to the extra charge, you can charge them. But if the original agreement never put them on notice and you also never told them about extra charges when they asked for extra work, you cannot charge them, because in that instance, they never agreed to pay more.