You shouldn't need to open the headlight up.
The best way to tackle this problem is by doing some recon work first, and actually looking at what you actually have versus what you don't have.
Jack the car up, take a wheel off, remove the inner rain guard and you will have excellent access to the back of the headlight.
Pop out the top cover(globe) and inspect the setup - is the HID socket still in there and connected (to the ballast), is the ballast still bolted to the bottom of the light.
If these questions are answered with a 'YES', then all you have to do is get power to the ballast, remove the adapter plate for the halogen globe and fit some HID bulbs.