Animating Frequancy

[Wlashy]

Hi Tyflow Community, 

Quite new to Tyflow and loving it so far. Amazing software

I have a question to you all. I am trying to create a simulation where the particle birth changes particle shape over time. 

Eg the simulation starts birthing cubes and finishes birthing spheres. I was planning to do this buy adding 2 particle shapes, animating the cube frequency from 100% to 0 and the sphere frequency from 0 to 100%. It seems however I cannot animate the frequency amount? 

Any ideas I could go about this?

Any help would be appreciated. 

Thanks