Why?

They've remoulded the mouth (not the head) to make it more accurate to the original 1984 G1 Jazz because previous Jazz reissues have featured the G2 "sh**-eating grin."

Encore Ratchet and Ironhide on the other hand already do feature their original G1 heads. For better or worse, Encore Ratchet and Ironhide's heads are very accurate to the original G1 toys.