If you start eatint better will the hardening of the arteries caused by arteriosclerosis go away?

Yes and no: Dietary changes aimed at substantial reduction in cholesterol levels is required. Most people can achieve modes cholesterol reduction switching from red meat to fish. Most need medications. Switching to a vegan diet can have a big effect on cholesterol. I'm not sure there is evidence that this causes an actual change in the artery, but rate of heart attack and stroke is less.