Antaran,

I actually don't own Carrara ERC; I just read up on it over at the DAZ forums. It was 1/2 off the other day, but I missed the dang sale!

Another really easy solution is to use Carrara's Multichannel Mixer.

If you're using a texture map in the Glow channel for the planet's city lights, simply use that shader tree as the first source in the mixer. (The whole tree) Copy the first source into the second source and get rid of the lights. For the blender, choose "Value 1-100"

Once everything's set up, you'll be able to animate the blender's slider to transition between the two different shaders. (One with the lights on and one with 'em off)

After that, it's just a matter of playing around with the keyframes to make the transition coincide with whatever lights you're using. 

The most important thing to remember is that there's NO CHEATING in 3D!!!

JT