Wear gloves.
The amount of spray you're risking isn't enough to be all suited up, but it will be more comfortable and a bit better than if the -40F liquid does pfft a bit on your skin.
MAKE SURE THE VALVES ARE OFF BEFORE DISCONNECTING!!!
NEVER TURN ON THE MAIN GAS WHEN THE SMALL BOTTLE'S VALVE IS OPEN AND NOT CONNECTED!!!
WEAR EYE PROTECTION!!! Your eye likely will NOT be happy if -40F hits it.
Not a fire risk, but a physical, send you to the hospital risk of a violently whipping around valve risk.
Filling a different kind of bottle a bunch of years back, I had a "helper" who mixed up the order of things and told me the big tank was empty.
As he turned on the big tank, I tried the grab the valve but too late. I took a valve to the face. Fortunately the glasses protected my eyes, but I had two mighty cuts on the cheek and eyebrow that required repair.
Be smart and be careful.
Also know that propane is heavier than air and will collect at your feet like puddles in dips and stuff. We're talking about big leaks, not pffts here and there.
End of safety message :)