I'm not sure why anybody would want to switch from a G Pro to a G2, or any of the upcoming flagship phones for that matter. The only real competitor to the G Pro is the Note 3, and unless you're made of money, the G Pro is a considerably better bang for the buck. The rest...do you seriously need that Snapdragon 800? That's about the only advantage they all have, and you're actually downgrading on the screen size. So many of them won't have removable batteries or SD card slots as well (Xperia Z1, Nexus 5, G2, Xiaomi MI3). I'm actually amazed LG took out the removable battery and SD slot from the G2, given the G Pro had both, and the G2 does not sport some kind of premium unibody design like the Optimus G/Nexus 4 had. The Xperia Z1 has a better camera by pixel count, but again do you really need that?
Of course if you want a physically smaller phone...than by all means, trade in. I feel set for a while myself, especially once development ramps up just a bit more.