You can also do a keg to keg transfer between the bev out of the sanke (commercial) keg and your corny. I did this to save space, went from a half empty pony keg to a corny. Steps:
1. Sanitize target corny
2. Put a qd on the other end of your sanke out bev line, but don't attach yet.
3. With the top on the target corny, give it some gas and vent to remove oxygen.
4. With the target with no pressure, hook up your sanke to corny bev to bev line. The beer will start to transfer. Keep your gas applied to the sanke source.
5. Every 30-60 seconds, vent the target keg a bit. This keeps the source at a higher pressure and the beer flowing.
You can tell how much is in the target by the condensation on the outside of the keg, or by weighing it before the transfer and during.
This method keeps oxygen out, keeps the same carbonation level, etc.