Other than the wattage being different (9006 are 55w, 9005 are 65w), if you hold the 2 bulbs up side by side, you will see the surrounding tabs are a little different. This is so you don't put the "wrong bulb" in the "wrong" socket. But since the tabs are plastic, you can use a file to ground down the plastic on the bulb, or even some wire cutters to snip the plastic.
With about 2 minutes of work, you can make either bulb fit in either socket.
My experience is with the regular halogen bulbs. I am assuming the HID's are the same.
Check inside the bulb connector too, I know that some are different than others, can be fixed with razor blade and steady hand though.
That's right...I forgot about that. There are plastic tabs inside the connector that need to be ground down. If you have a soldering iron and you are careful, you can also heat up/melt the plastic and yank it out.
you can go to the 9005 highbeam in a 9006 lowbeam but you CAN NOT go backwards. the base that the o-ring sits in is slightly smaller in the 9005 in the 9005 which allows this to fit in the hole. the 9006 is larger at thebase and will not go backwards into a 9005 hole. you also have to modify the inside of the connecter. there are two tabs on the top side of it, just take an x-acto knife and file them down flat.
06 GMC Sierra
"When you do things right, people wont be sure you've done anything at all"
I realized the amount of destruction I will have to do to my lights in order to make the 9006 fit in the 9005 housing. So rather then destroy a set of 600 dollar lights I'll spend the money and pick up a new set of HID bulbs.