Actually, the responses in this thread have been generally impressive. I am used to seeing a barrage of "no way, you're ridiculous to ask such a question, I'll prove it with a blind test", etc., etc.
I think it can be summed up, although many people have stated it - some a bit more direct than others:
1. If the sound improvements you are looking for are affected by physical changes that occur in the headphone materials, then yes, there is a possibility that longer wear will change or improve that sound.
2. On the other hand, if the sound changes you are looking for are affected only by the electronics involved, then no. The measurements will tend to be the same months or years later as they were when first measured.
The real question is, what components make up either 1, 2, or a combination of both in a particular headphone?