Is there any reason I shouldn't replace my HID bulb myself?

Be sure not to touch the bulb with your fingers. Food service gloves are fine.

If the 12 volt battery is disconnected, nothing electrical can happen. (There are interlocks requiring 12 volts before the High Voltage can flow)

 

Welcome to PriusChat! Changing the HID lamps is a DIY job, and easier if you have small hands, so that you don't necessarily have to disassemble the front of your car, but there are some great videos on youtube as to how! Here's just one:

 

Pull the bumper. Remove light don't touch the bulb or let it touch the side of the light of it will only last 2mo

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I went through this 3 or 4 times a few months ago and the best way was to take the bumper loose to where it falls forward then the bolts for the headlight are right there. It took about 10 minutes. I'm not sure why you would have to take the 12v battery loose but wouldn't hurt anything I guess. I ended up replacing 1 bulb and 1 ballast.

 

Hmmmm...

13.5V Aux battery.
HID works in the kilovolt range (in excess of 10,000v)

My Spidey sense tells me that there's more at work here than meets the eye.
I'm juuuuust guessing, but there might be a capacitor or a coil in there somewhere making up the difference, so the "just disconnect the battery" theory might not be completely true. Capacitors are pretty good at storing energy (sorta the whole point.)

However (comma!) since HID lights only need the high holtage to kick-start the (Xenon?) light, then I'm guessing that unless somebody plays a nasty prank on you while your sweaty salty mitts are grubbing around behind the bumper then you'll be fine.
A cattle prod (or a Taser) works in the same voltage range, so even if you get zapped (unlikely) it's probably not going to be lethal unless you have a preexisting condition like a pacemaker.
I'd keep the car OFF to be sure (and put the fob inside the house, and keep pranksters and Grandchildren away from the light switch!)
I'd also wear wubber gloves but I would not disconnect the Aux battery.

It doesn't really gain you anything and it will wipe out all of your mileage info,clock, and presets.

Good Luck!!

Replace it yourself????
Yes.

 

They call the coil a Ballast

 

Removing the fuses will remove the supply voltage for the HID bulbs, and that's probably the best approach rather than yanking the battery leads or a main fuse.
Just make sure you pull the right fuse.

I wouldn't bother, but that's me being me again.
This isn't going to be a really big problem.
Glass is a pretty good insulator (even against circuits in the tens of kilovolt range) and the bulbs are made to be replaced by the average consumer. Dealers charge a bazillion bucks to change them out because (a) they're crooks and (b) because the bulbs can be kinda tough to get to.

Think about it.
If people were getting zorched by these things even semi-regularly then the HID manufactures AND the auto makers would be getting their butts sued off by slip-and-fall lawyers, and the lighting system would be in a $2400 sealed unit that you have to have a federal license to replace.

Those of us who are old enough to remember film photography are familiar with the "whhheeeeeeeiiiii" of a charging flash circuit. Once the circuit is charged the battery is done doing its thing and you can take it out.That does NOT mean however that the flash is de-energized.
That's what I meant by hinting that removing a fuse or a battery might give you a false sense of security.

You can however replace the bulbs no sweat......even if you DO sweat.
You have to wear rubber gloves to handle the bulbs anyway (not about to segue into wearing latex in a post about batteries and being safe!!! )

Good Luck!