Animating complex cloud


I have got the assignment to animate this complex looking cloud shape. I need to show how it morphs from simple sphere into this complex final shape shown in picture.
I also have exported middle states between sphere and then more and more complex shapes until this final one on picture.

Models are in x3d with vertex colors. I need the this animation to be displayed in Unity 5. My first try would be with shape keys. Does anybody have a tutorial or some experience with such complex shapes (300K) ?

P.S. I have already checked, and Unity does splits meshes over 65K tris into parts, but animation plays normally. Also, I have a shader to display vertex colors in Unity :smiley: