Hehehe you made my day, no one who recommend sai for animation.



Right now the best frame to frame animation software is gimp (the easiest to handle), in the past there was a program called jasc animation shop but was discontinued.



The best way is the animation of transformation, the transition is more natural, saves so much time and labor. About this the market is dominated by adobe with its flash, but I've read very good recommendations of toon boom tools, however I've not tested yet.