An estimate can change to anything as long as they call you and tell you what the change is going to be. You can choose to pay it or stop the work there. Like with any significant work, you don’t know what is going to be found or needed til it’s torn down. When they do a diagnosis for an hour or two of labor, they’re not tearing down the differential to figure out what exactly is needed. That’s why it’s an estimate not an exact. If you feel you’ve not been properly notified, then you need to run it up the chain. It’s a franchise. That shop has someone to answer to.