no matter what, purge the keg.
*BUT*, i don't leave my kegs hooked up to co2.
I mounted a bike tire valve on a gas-in fitting. 4 to 4.5 gals gives me enough headroom.
foosh it (with a tire inflator valve) up to about 20 psi.
I use air compressor QD's (300psi max) on my gas lines, so I can hook/unhook in 1.358 seconds.
natural carb if you like, but there's no reason that you have to.
you can see why i have to do that, here: