Analysts predict that the very last time you will be able to play any game on a GTX 770 on Medium will be July 12, 2017. If you install the GTX 780, then the deadline for playing games on Medium will be extended to January 27th, 2018.
Thereafter you will be able to play all games in Low on the GTX 770 until April 24th 2020 and on the GTX 780 until May 19th 2021.
If this turns out to be untrue, feel free to claim your refund from me.