If you have the proper connectors, you could do it under gas. You would want to chill both kegs to reduce foaming and be patient for the foam to subside.
Probably easiest to tap slowly into a chilled corny. Like filling a big mug.
Some CO2 will come out of solution, but you can always replenish it.
You could fab together all of the necessary connectors to transfer in a closed fashion, but that'd be a little more effort and $. Yould have to open the pressure relief valve on the target corny.