I'm not trying to speak for the CS team... but chances are if there is a 767III coming(which I've heard nothing of), it won't be for a very long time considering the 767II is unfinished.
As for the free upgrade, I doubt it. They've not offered any free upgrades as of yet. Except on the 737-200 for previous P3D3 owners. Besides the FSX 767 and 767II are completely different so having to pay full price is understandable. I imagine a 767III would be the same.
I don't know where you heard a new 767 is coming but I've not heard anything about it. I doubt one is even being considered. The current 767 just came out less than a year ago and as I said its unfinished.

I would definitely get the 767 now cause it is a very nice aircraft that I've not found any major complaints with. I just wish the GE engines would come out but I'm perfectly happy with it as is.