Phil,
It depends upon the reason for the pending replacement. I can only speak about replacement due to aortic stenosis. In the case of a stenotic valve (doesn't open all the way), eventually the left ventricle will enlarge due to the stress the work of pumping through a small opening puts on the muscle. They have standard measurements for the size of the ventricle and will recommend valve replacement if your ventricle approaches these limits, as they have observed that once the ventricle goes past these limits it may not remodel back to a normal size after valve replacement. If the ventricle remains too large, it can restrict the heart's ability to pump fully, which would result in much less than the best recovery.
There is probably a different situation that would result from aortic regurgitation (when the valve opens but does not close all the way), but since I never had any more than trace regurgitation, I'm not familiar with the situation. Perhaps one of the others can cover that one for us.
