Most studios create new character designs are
because Japanese fans usually tend to like the 
improved designs over the originals. 
Unfortunately in many cases, the new designs 
don't measure up to the original designs.

Also, the new designs can create a brand new line 
of merchandises.  Let's take the Escaflowne The 
Movie for example, since it has new mecha designs,
Bandai can now release new model kits and toys
for it.  Same thing to the other manufacturers,
now they can create a bunch of new merchandises 
based on the new character designs.