Upload & Sell: On
I seriously question the concept of trading in the 28-70 on a 24-70. There is always loss when trading/selling/buying, and I very much doubt that you will see significant IQ improvements in the 24-70 v1 over the 28-70. If the focal length range 24 - 28 makes a huge difference to you, then go ahead, but you won't see a lot of IQ difference between the lenses.
In general, good glass holds its value far, far better than camera bodies. The 6D is a fine body; if you don't have specific complaints that the 5DIII would solve (and if the large hunk of cash it would take to make that trade is an issue for you), I certainly would keep the 6D.
Figure out what problem you're trying to solve. If it's low light, the you might think about adding a fast 85 or 135 to your kit (or to learn how to take and process images at higher ISO settings).