Probably the best way is to make a "jumper cable" type set up. That's what I do.
I've never moved to a 1/2 full keg, but I "jumped" into empty kegs all the time. I like to do it when I take a keg on the road, to leave the sediment behind in the first keg.
Anyway, how it works is this- black QD, beerline (about 3' is good), black QD. That's it. Then, put the first black QD on the first keg. Vent the second keg so that there is no pressure on it. (Take the receiving keg OFF the gas!) Turn the psi down to about 2psi on your regulator. Then place the second black QD on the "out" post of the receiving keg. You're going from "out" to "out" with the black QDs. The beer will start to flow. Keep pulling the pressure relief valve on the second keg. That's it! The beer will foam slightly, but not as much as if you were racking or using the faucet.