I am currently in the process of restyling such an illustration, and I would like to get it back to being an open path (that I can style by the stroke).
Is there an easy/convenient way to do it? I tried with a cutting the path to two open parallell paths, and then blending them, but it didn't capture all the twists and turns properly...
What twists and turns do you want captured? An average? It looks to me that the simplest thing would be to simply grab the Pencil Tool and draw along the yellow line. Sometimes the easiest way is to resort to the old fashioned technique of drawing (no disrespect meant!).
I tried with a cutting the path to two open parallell paths, and then
blending them, but it didn't capture all the twists and turns properly...
Fraxinus,
Blends work by averaging the positions of sequentially-corresponding anchorPoint and handle coordinates. If the two paths being blended have different number of points (as yours most probably do), a Blend is not likely to give you the kind of shape-accurate result you are trying to achieve.
Best bet is to just redraw the paths. You could tweak and twiddle with the paths you are trying to blend, adding points here, removing them there; but you'd end up doing more work than just re-drawing the centerline anyway.
In the future, if you think you are going to need to "revert" a piece of artwork in which you have outlined the strokes, save a copy with the basic paths as a separate file before doing the destructive path operations.
JET