I apologize if this has been addressed already, but here goes:
1. If your negative is simply overdeveloped, then your contrast will be too high. If you bleach such a negative, the very first thing that will be bleached out are your shadows. They will basically just disappear, while your highlights remain still pretty dense. Now you will have a negative that is still of very high contrast, but with less shadow detail than you had before.
2. If your negative is overexposed, on the other hand, and the entire tone scale of the recorded scene is slid up the tone curve of your negative, then you can bleach with good results to reduce the density of the negative in general, without losing shadow detail; shadows will just slide back down the curve to where they normally live.