Good question, and a point that I had completely overlooked.
Halogen and HID headlights use different harnesses and wiring.
Halogen is pretty simple: A relay sends power to the low beams. Another relay turns on to power the high beams.
HID is a little more involved: The low beam relay turns on, a new relay sends power to a ballast and/or igniter, then special high-voltage wires go to the bulb. The high beam relay output adds an adapter that sends 12V to the solenoid that lifts and lowers the shield for high and low beams while also keeping the stock high beams working.
The BCM also treats the HIDs differently, not only by how the high beams are turned on, but in how the DRLs are illuminated. HIDs usually (always??) have separate DRLs, whereas halogens usually serve as DRLs at reduced power. Some halogen DRLs use the high beams and others use the lows.
For your purposes, if you are already going to be doing a measure of work and repairs to get the car back on the road, the wiring modifications should not be that much of an issue.
When I modified my right housing to accept the HID module (reflector, bulb, etc.), it also required some wiring modifications. The most important one was to modify the wiring at the BCM that interrupts the feature for automatic headlights that turns them on and off based solely on the amount of light falling on the sensor. By snipping and rerouting only a few wires I was able to allow the BCM to continue to warn me if the lights were on when exiting the vehicle, yet not turn them on or off based on the sensor. To turn on my headlights I now simply turn them on manually with the headlight switch.
Under the hood, I used the 12V going to the low beams from the stock relay to power an added relay (part of the kit) for the ballast/igniter and the high beams to operate the shield. I used a harness supplied with the conversion kit. What I have now (on the right side only so far!) is 1 low beam on lows, and 2 high beams on high. In factory mode, the halogen low beam remains on during high or low beams, then adds the high beam when on high. On HID, the bulb also remains illuminated during high and low beams, but the difference is when the other bulb is added for high beams, the shield moves out of the way to allow more light from the HID, making it a true 2-bulb high beam rather than just a 1-bulb add-on. When I'm done with both sides, I'll have actual high beams instead of 2 lows and 2 added highs just by virtue of the fact that the cutoff shield stops blocking the high beam light from the HID.
If all of this is kinda making your head swim, maybe this will help:
Stock low beam relay -> HID relay -> Ballast/Igniter -> HID bulb
Stock high beam relay -> Special adapter, branch 1 -> Bi-Xenon cutoff solenoid
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . branch 2 -> Stock high beam bulb
In my cars for over a decade I have modified them to use the stock turn signals as DRLs. It is a simple module that I designed and built into a small project box. In the Malibu I've been able to do all of the wiring under the dash at the BCM instead of having to modify wiring under the hood, which is much more exposed to heat and water.