Ah ok, it's been set up as a TBOE, that's actually a little more efficient but with pumped systems these days I'm sure the benefit is minimal. It probably came from an old single pipe system.
If you do need to change it then the best way to do that would be as @muggles suggests. Good dose of heat, wire brush the threads and any paint around the tapping, keep heating until all the paint is removed then a large adjustable with plenty of leverage, just go smoothly with the pressure. Be prepared for something to go.