Then you shorten iterations if you need to change priorities more often, or you stop the iteration and start a new one. Waterfall created a scenario where requirements couldn't be changed without significant effort. Agile creates a situation where those managing the backlog can change requirements whenever they see fit. And developers, next time an iteration is planned, take the top items from the backlog that fit into their iteration capacity.

The side effect of shorter iterations is that requirements have to be broken down to fit inside the smaller iteration, so there is a price to be paid by those managing the backlog and generating requirements if change needs to happen quickly.

Agile methodologies don't enable change as needed, they create a structured way of handling inevitable change. And when the backlog is adjusted to accommodate that change, the consequences of that change to budget and delivery date are easy for everyone to see.

If you don't have controls in place, then there is no understanding of the cost of change. And without understanding the cost, good cost-value decisions can't be made.