I believe it is o.k. if you drop the pressure down to 2 psi and purge the gas in the keg first. The key is really slow.
If foaming is a big concern you may try this: gas up the empty keg first and then let it hang for a minute, and then purge the air. Then purge, and connect the full keg to the empty without the gas line hooked up. It should start transferring on it's own in a little while. If you hang up the relief valve on the empty, it will transfer really slow, maybe an hour or three or longer.
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