The cost of doing this would be just shy of under $1000.
I can't remember everything about this process, but what I remember is since Optimus is back in the VX model you would be paying for not just the card, heat sink, cooling adjustments (bios related temperature thresholds to fan speed settings, voltages, link adaption and pairing) into bios flashing to accustom the 670MX.. possibly even the VX model motherboard. Not to mention you will need to modify drivers to install them as support for the 670MX isn't quite there yet.
In all honesty, it would be better to sell or return (if within policy) what you have now, and purchase a VX model instead.
Believe me, I would like to do this upgrade as well. But the cost doesn't outweigh coming out ahead of it. You basically will be paying more for less.