Fire material to smoke material

Hi, I know how to add a material to a particle system, like a fire texture, but how do I change it to a smoke material later in the particles life?

You could use a sprite sheet that starts as fire and then turns to smoke, but the standard method is to just instantiate a smoke particle system in tandem (or after a specified time) with the fire particle system.

here's a great tutorial that covers quite a bit:

http://forestjohnson.blogspot.com/2007/12/particle-effects-tutorial.html