Hi, Arterial wall-thickening and reduction in blood flow is extremely difficult to reverse. Also, with the "plaque buildup" the flexibility of the arterial wall is much lower. This is why it is sometimes called "hardening" of the arteries. Tests with children (on bad diets), here in Australia, have shown complete plaque reversal (with exercise and better diet) in their arteries. These arteries are of course growing and much more flexible - unfortunately this does seem to apply to adults, especially older folk.
A totally fat-free diet MAY produce a very small plaque reduction (e.g., see "Reversing Heart Disease" by Dean Ornish), but this is a diet so severe few could stay with it.
High doses of very strong statin drugs (eg "Crestor" - by far the strongest statin) MAY also produce very slight reductions in plaque deposits. This, whether rightly or wrongly, leads many of us to take high (excessive?) statin doses - much higher than needed for healthy cholesterol control. There could well be other risks associated with these statin doses.
Exercise is adopted by many with arterial disease but again results are mixed and inconclusive. For most exercise certainly helps improve physical stamina, and for some can raise or even eliminate the threshold for the onset of angina. Candidates for PCI (stent) procedures (to open up heavily-constricted arteries) have, in some studies, found that with appropriate exercise, the main symptoms of chest pain and shortness of breath have decreased or even disappeared, to the point where they no longer require the PCI procedure. As far as I know this is not the rule. For others exercise makes little difference.
So, for many the thickening is a rather nasty progressive disease. But with MAJOR lifestyle changes (diet, exercise, medication, smoking and drinking cessation) I believe the disease can be slowed dramatically, or even halted.
The other point is that every case is different, and patient's responses are different. I know people who still smoke, keep on their old diet and do no specific exercise. Making the lifestyle change is just beyond them, or maybe they have succumbed to their fate.
Drugs that remove the plaque are being researched all the time. It could happen any day - we can only hope.