I agree with workmonkey's comment about the graft areas likelihood of re-dissecting is extremely low, if not impossible.
That being said, the aorta itself is much larger than a valve graft, and it is those areas that are not protected from future dissection or aneurysms, especially if you have a connective tissue disorder.
In my case, my aorta dissected from the aortic valve, all the way down through the abdomen, then branched off down to my left knee cap area. This happened in March of 2000, I had to have emergency surgery, and they gave me a 2 to 3% chance of surviving it, they even told my wife to prepare and say her goodbyes to me.
So far no additional changes to the dissection.
Since then, I have had to have a 2nd aortic surgery to replace the mechanical valve and graft installed in March of 2000 when I dissected. The reason for this was due to panus growth which was preventing the valve from opening and closing.
My 2nd surgery in Sept of 2010 involved replacing the valve, building a graft for the valve, replacing the aortic root, fixing an aneurysm they found on the side of the arch, and then to further support the dissection, they preformed a frozen elephant trunk procedure, which is really installation of a stint with 20 cm of Dacron tubing attached inside the aorta. The stint is sewn in to the aorta at the top side, and the Dacron tubing extends downward in the aorta. This directs the pressure of the main blood flow through the aorta through the Dacron tubing, thus reducing the impact of the pressure against the aortic walls in that area.
14 years after my initial surgery, all is still stable, and I remain an active person.
Rob