I put it on a cotton towel (not synthetic cos it'll melt) in the oven. done about 14 headlights this way, pulled it apart, painted the insides or installed HID upgrades and then put it back together. If you leave the silicone sealant pliable, then you can just use a heat gun to make it flow again and reseal the glass when you put it back in.
If you want to remove the silicone, then bunnings sells this putty that reacts with silicone and sealants and makes them go gelatinous, which makes it really easy to just brush it out after and reseal from scratch or install fibreglass/plastic clear lenses.