Animating Frequancy

[Wlashy]

where the particle birth changes particle shape over time

Not sure what you mean by this? 
How can the particle birth change particle shape? 

As for animating frequency... if you mean, animating frequency of particles being born, you can use regular birth operator, change the method to "per frame", and just animate that parameter.

Thanks for replying d4rk3lf, Ill try and explain better.

Example, I have a car model, from the exhaust its starts with carbon being emitted from exhaust (cubes), as the animation progresses, I want bubbles (spheres) to be emitted from exhaust. What is the best way to do this?

frame 0 - 100% cubes
Frame 100 - 50% cubes and spheres
Frame 200 - 100% spheres. 

Cheers

Rob