If they are sealed with silicon then you need to heat it up to pry it open. Oven is the commonly used method as you have found but i prefer to use a portable fan heater as ive seen some pics of headlights that got melted in the oven so i didnt want to risk it when a decent heater will get it warm enough to open, also people in my house probably dont want car parts in the oven lol.

To remove the old silicon use turps(or another sutable solvent) on a rag to rub it off then reapply new silicon to prevent the housing from getting condensation.

What are you using to clean/polish them?