Try a bigger wrench.......a Stilson Wrench....the classic "monkey-wrench" tightens it's grip as pressure is applied.....add a section of pipe to the handle for more force.........
"Left-y Loose.......Right-y Tight....."
If the pipes not too corroded .....there are bubber boots available to join the old and new pipes.....without removing any additional pipes.......
If you have to CUT the fitting........I'd use a dremel with a fiber cut-off wheel to cut (just)thru the nut allowing the nut to be removed and then the pipe changed..........