For what it's worth, there are differences on the label. Old pre-flag chalk does not have the trademark symbol, and uses "ILL." along with the zip: "60612"
The new pre-flag chalk has the trademark symbol and uses "IL" with no zip code.
If the formula hasn't changed, it could very well be that the ingredients have changed in some way. I do know that the label changed at least once prior to the flag being removed.
I will say this, comparing the old stuff to the newest I have, it FEELS different (to the finger, my cue doesn't seem to care).