As given iTunes is not currently supported on Windows 8, however this is the approach that I would recommend when the time comes.
- Backup the library with this User Tip.
- Deauthorize the old operating system.
- Install new operating system.
- Restore the backup to your new system using the same tool used to back it up.
- Install iTunes.
- Keep your backup up-to-date in future.
This should give the new system a complete clone of the library that was on the old one. As far as iTunes is concerned this is still the "home" library for your devices so you shouldn't have any issues with iTunes wanting to erase and reload.
If you have an iOS device that syncs with contact & calendar data on your computer you should migrate this information too. If that isn't possible create a dummy entry of each type in your new profile and iTunes should offer to merge the existing data from the device into the computer, otherwise the danger is that it will wipe the information from the device.
If your media folder has been split out from the main iTunes folder you may need to do some preparatory work to make it easier to move. See make a split library portable.
Should you be in the unfortunate position where you are no longer able to access your original library, or a backup of it, then see Recover your iTunes library from your iPod or iOS device for advice on how to set up your devices with a new library with the maximum preservation of data. If you don't have any Apple devices then see HT2519 - Downloading past purchases from the App Store, iBookstore, and iTunes Store.
What does the backup tool accomplish that I'm not already doing? My iTunes library lives on an external hard drive, and I want to keep it that way, so I'm not seeing what this backup tool actually does for me. I have the entire external drive backed up to a cloud-based backup service besides.
Considering the above, I therefore still don't have any idea what the proper order of A, B, C, and D should be.
I'm glad to hear you have a backup. Too many people don't
You've proposed 4 steps:
A) Connect the drive
B) Open iTunes
C) Change iTunes' path to the drive
D) Log in to iTunes
The order is correct but B&C are a slightly different operation. Hold down shift as you launch iTunes, click choose when prompted to choose or create a library, and connect to the library sitting on the external drive. The success of this operation depends on the library database existing on the external drive, not just the media folder. Again it is not uncommon for people to misunderstand which files they have preserved and find they have their media but no iTunes database. The link make a split library portable should explain how to get your library into a portable form if it isn't already so that everything works properly when connected to the new system. I realize that you've said your library is all on the external drive, but even if the extra information doesn't benefit you it may help the next person who runs across this thread looking for solutions.