If you are replacing the soft line...
You can slit the part the hard line screws into. That will help let off the tension and hopefully save the hard line any damage.
You need a dremel tool and # 420 cutting disks. You can get these separately or as part of a three grit disk pack. You can get the cutting disks at W/Wall mart and many home stores. (Oddly, the #420 "Heavy Duty" wheel is the middle weight of the ones available...)
Cut along the side of the fitting long way. (Not across like you are sawing it in two.) Cut most of the way down to the threads on the line nut. You usually do not have to cut all the way to the line nut. The fitting will stretch/crack along your cut when you try twisting it again.
As many people have observed these small cutting wheels can be fragile. It helps to use both hands and try to find a grip that lets you brace your hands against something.
On the line nuts... You should be using Flair nut wrenches. If you don't have those the line nut can round off easy. If you use vise grips you need to adjust them as tight at you can but not so tight they squeaze the nut out of round. (I've seen guys do that with big 'grips. The nut may not fit in the new line if that happens.) Those Robo Grip or similar pliers might be better on the line nut.
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Screamin' Yellow Zonkers... If it's Screaming and Yellow, I aint eatin it.