I can get the outer surface of the headlamp assembly clear by sanding/polishing but the plastic contains a lot of microcracks which resemble cob webs and which go all the way through the plastic.

My question is can I buy new halogen assemblies and change out the halogen components for the HID components I have (bulb socekts, harness, etc.)? I believe I have seen a youtube video where someone opens the light assembly with a heat gun to install halos so that is where this idea comes from.

The HID sedan replacement assemblies are VERY expensive ~ $400 a piece.

 

Not necessarily. You can swap the plastic lense covers pretty easy.