All this talk of snatching. You need to appreciate the difference between doing it on a sealed system and an open vented system. Once a sealed system has lost its initial pressure and the water in the system is at atmospheric pressure, the existing water in the system is a vacuum. Unless you introduce air into the system, in theory the water can not be released from the pipes, to see this in action, place a clear straw into a glass of water, hold your thumb over the straw and pull it out, most of the water will remain in the straw until your remove your thumb. The same happens to water in a heating system.
So as long as you don't allow more than one outlet to be open, you can quite easily change a trv on a sealed system, with minimal mess (always protect the surrounding areas though!).
You can not do the same on an open vented system, you need to create a vacuum by bunging the open vent and cold feed at the header tank. Also, never bung the feed and vent to a cylinder, it can cause the cylinder to implode!