I'd think if the blade is very large or thick, trying to 'straighten' out the recurve by hand, with sandpaper on soft backing, might be an arduous task. Might be a job for someone experienced with powered sharpening tools, such as a belt grinder. Even then, I don't think I'd do it (personally), unless you just can't live without it.
I suppose a recurve could be gradually 'straightened' over time, grinding away the portions in front of, and behind the recurve, with multiple edge-leading, acute-angle sharpenings on a flat bench stone, like a coarse diamond. I think if I were determined to do it by hand, that's how I'd start. Once most of the recurve is gone, then using the sandpaper to convex the whole edge would be simpler.
