I've never heard of a "spring overseed." Maybe that's because I turn off the volume when I see springtime Scott's commercials. I've heard of fall overseed, but spring is the wrong time of year to seed fescue. Why? Because spring is right before summer and the new seedlings will have no chance of making it. Late August is a much better time to do the job. Can you nurse it along until then or is it too thin now? If you have to do it now, then you have to, but you already know the possible (likely?) consequences.