Adding effects to a 3D model only

Hi. I've added a 3D model to the top of this tower which opens up projecting a shadow on the tower.

What I want to do now is change the look of the model with noise, colour correction etc. But I want everything else to to stay the same. How can I change just the model? I can move it to it's own comp and add effects but then the shadow projection no longer works.

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