I have a 2009 XF and I had the headlights modified a couple years ago to black insides. Love it but last few months, the insides are becoming a bit cloudy. Not too bad but not crystal clear like they were at first. We figured out that because we had to heat cut them open, the fumes from the melting plastic must be what's finally turning hazy.
I'm debating finding a pair of used Halogen lights and trying again to heat open them. I didn't try before because I saw some threads online that the glue us too strong and it damages the housing but now IV expound out it takes higher heat than most glues. Still, there's a chance of damage.
My question is: can I retro fit the halogen fixtures with HID, either from my existing lights or an after market kit (I already have)

 

I don't see why an after market kit wouldn't work, You may have do some persuading, but most HID kits are based on the type of bulb used, not the housing they fit in, order the HID kit for the bulbs you have and they should fit. It would then be just a matter of finding a place for the ballast boxes.