What do you mean by morphing a rectangle to a sphere keeping it's volume?
I can think of these options:
1. - A rectangular surface to a sphere (keeping constant surface area)
2. - A box to a sphere (keeping constant volume)
3. - A rectangle to a circle (keeping constant area)
I've done number 3 since it's the easiest :P
There are several ways to morph. If you divide both surfaces or curves into points, find the average position and then interpolate through them a surface or curve, you'll only get an approximation of a circle or sphere. To get a perfect circle or sphere, you should get the control points and their weight and also interpolate their weight (this is my deduction, i could be wrong), but this is not exposed in grasshopper. The next best thing is doing a loft, as i did in the screenshot and extracting sections.
The surfaces would be planar. Imagine defining any planar surface from a curve. I want to be able to manipulate the original curve using the control points and have the geometry of the curve react and move in a way that preserves the area of the surface.