When you say modify an object on a motion path what exactly do you mean? If you could explain your thought process a little more it would help to pinpoint just how we can be of assistance.

For example, if while a cube was floating down along a path you wanted to scale it in the y-axis, you should be able to key frame that into its movement along a path.

Changing the direction of the path is as easy as moving cv's on your curve.

If you want more explained please ask, but if you could further explain your thought process or what you are trying to do please do so first.

Thanks!
-Alexander