Transition Layers between 2 Head Swap sets

Hi,

I'm sure the following is answered somewhere. If so, can anyone point me kindly to the tutorial? I can't find it.

Problem:

I have two head groups in a swap set: 1) Frontal 2) Quarter view to the right

Each head is tagged and its features (eyes, mouths, etc...) perform as expected.

I trigger the quarter view with a key

NOW: how do I key trigger a smooth transition from frontal to quarter view? I would need to cycle three layers in between, but obviously if I just key trigger the quarter view it comes on immediately.

Thanks,

Johannes