the safest way of doing this is to drain the whole system. upon refilling you would have to vent air from all the radiators and the pump, there is usually vents as part of the pipework next to the hot water cylinder too.
what the plumber you used before did was to create a vacumn with a "bung kit", bunging the open vent and the outlet from the header tank, also tying up the ballcock to the header tank or turning the mains water off just in case the vacumn doesn't hold. there is a real risk that the vacumn may not hold so its best to work as fast as you can. you would also have to use compression fittings when re-arranging your pipework as you cannot solder with water in the pipes.