From a super quick Google search..
The second limitation of potassium sorbate is the length of time it is effective. Once added to wine it stays in the desireable form of sorbic acid only for a short time. Over time it breaks down into ethyl sorbate which can add notes of pineapple or celery to your wine.
and
Potassium Sorbate does not kill the yeast at all, but rather it makes the yeast sterile. In other words, it impairs the yeast's ability to reproduce itself. But, it does not hinder the yeast's ability to ferment sugar into alcohol.
Potassium Sorbate puts a coating on the cell wall of each individual yeast in such a way that budding or multiplying is next to impossible.
Based on my interpretation, it sounds like there would be a point where if you introduce enough new yeast that you'd be able to overcome the effect of the sorbate (i.e., there is a limit on the number of yeast cells the amount of sorbate you previously added can "coat"). That doesn't even account for the break down of the sorbate over the past year.
That is the best I can figure from a scientific perspective. From a personal perspective, it sounds like a lot of work for something that might not even work. You'd have to expose it to oxygen to process it, add an unknown amount of yeast, and hope that it makes it all the way dry, when it somehow didn't finish in what we can assume were much more favorable conditions. I'd just make a new batch of dry wine and continue giving out the sweet wine to people who enjoy it that way.