So I have a simple bouncing ball animation, but I just wanted to something "weird" for the ending. Is there a way to change the ball's material on the fly, where after the last bounce it changes materials and physical composition from a rigid body(baked animation now) to a fluid sim? (within 30-40 frames)
No you can't.
But you can animate the visibility of objects so that on impact the original object is hidden and a replacement becomes visible. Object transparency is another property you may want to animate to ease the transition.
The render visibility is shown as a camera in the outliner, with the mouse over the camera for the object press
I to insert a keyframe for the value. The same can be done for other values like transparency in BI materials or Cycles nodes.
You may also want to render two sequences and mix the two together to get more control over how and when they replace each other.