It sounds like you're looking for mesh morphing, which is something that does not come out of the box. If your mesh is modest in size, you could however script this manually using the Mesh class. basically make sure that both poses have the same amount of vertices, and slowly move each vertex from the position it has in pose1 to the position it has in pose2.

If your avatar would have bones instead, this would become a lot easier.

One more way to do it (not sure if it suits you):

• put a material that can change its transparency (transparent\vertexlit, for example) on both avatars
• make sure the second avatar's Rendered is disabled
• fade out the first pose (by slowly changing alpha from 1.0 to 0.0 via changing renderer.material.color.a)
• enable Renderer on the second avatar (also disable it on the first one as there's no need for it now)
• fade in the new pose (slowly change alpha on the second avatar from 0 to 1)

I hope it helps ;).