Client approved everything… then asked to change half the project. How do you deal with this?

I'm running into this pattern over and over again. We go through requirements, mockups, approvals — everything looks locked in. Then development starts and suddenly: "can we tweak this section?" "maybe change how this works?" "this shouldn't be too hard to add, right?" And now I'm reworking parts that were already agreed on — sometimes hours of work gone, tbh. I get that clients don't fully understand the im