If the beer is carbonated, I like to get both kegs cold, and release all of the pressure from the full keg before connecting. The receiving keg should be purged of co2 with its PRV open. Connect the two and slowly raise the gas pressure. I like to run it slow to prevent foam.
Ideally, a spunding valve would be used to maintain head pressure on the receiving keg. I’ve been meaning to build one.
Make a jumper with 2 liquid fitting. Make one with 2 gas fittings. Have a purged keg, make sure the pressure is released. Have the keg with beer higher than the purged keg. Use the CO2 to start the transfer, then disconnect. Then hook up the gas posts. As the beer drains, the displaced gas goes on top of the beer in the first keg.
Saves CO2, trade off is the longer time it takes. Make sure your jumpers are long enough.
I have done a lot of blending beers (even fully carbonated ones) in this fashion. What worked best for me was to have just enough pressure on the full keg to push out the beer into the receiving keg which also had matching pressure on it to minimize foaming. It obviously takes awhile to transfer in this method, but you do maintain the carbonation levels without the beer foaming out of the PRV.
I can’t believe I’ve never thought of this. Usually I’m just walking over and popping the manual PRV every minute or so on the receiving keg. That adjustable PRV on a ball-lock QD is quickly becoming one of the most useful tools in my brewery.
I use this method with my sanitizers, I fill the keg with starsan completely full after cleaning it, then I’ll use Co2 to push into another cleaned keg. That way I have a completely O2 free keg for ready for filling, and the starsan keeps getting passed on to the next keg. I’ll test ph on it after about 4 or 5 kegs for effectiveness.