Yeah, I'd vent it down if it's in the keg. Release the pressure through the PRV , then close the PRV. Agitate the keg a little to break gas out of the beer. Let it sit for the foam to settle and the beer in the keg to equilibrate at its new level for a couple of hours, and repeat, until you get the carbonation down where you want it. If you rack onto carbonated beer, you will be agitating it, and you'll provide nucleation sites on yeast and hop particles, and it will be instant foam all over the place.