I clean-install everything, always. It's pretty easy with 8 and 8.1, though admittedly slightly more complicated than it was with 7.
0) If you haven't done so already, read your current key from the MSDM table. I use AIDA64 for this, but there are other freeware utilities that can do this.
1) Acquire an image. This is the hard part. Easiest way to find a legitimate image is to go to MSDN, click the link to access subscription benefits, and browse the library (this works even if you don't have a subscription). If you have a sub, then grab the 8.1 with Update image. If you don't have a sub, open up the details for that image and copy the SHA-1 hash. Now Google/Bing that SHA-1 hash to find a torrent or other download of that image (and of course, after downloading it, verify the hash using something like HashCheck). By searching for the hash (and verifying afterwards), you can be sure that you got a legitimate, untampered image.
2) Prepare a USB stick. The old DVD-to-USB tool that Microsoft released for Windows 7 won't work here because it formats the USB drive with NTFS, and NTFS is incompatible with EFI boot (and all systems that come preloaded with 8/8.1 use EFI boot). So you'll need to
format the drive manually with diskpart. It's a minor nuisance, but I only need to do it once, and whenever I replace with install image with a different install image, I can skip this step.
3) Copy all the contents of the ISO onto the USB stick (drag and drop the mounted ISO works, or unzipping the ISO using WinRAR or 7zip).
4) Use Notepad to create a "ei.cfg" file in the "sources" directory with the following contents:
(Yes, leave the Channel at "Retail". It doesn't matter, TBH.)
5) Boot and install from the USB drive. With the ei.cfg file present, it will not ask you for a key during installation.
6) When you first boot into the new installation, OOBE (the initial setup process) will ask you for a key (or it might not ask, if it already successfully detected yours during installation). Enter your key now, and it will activate.
The main problem that trips people up is that the key validation in the Windows
installer is very limited in what it accepts (I don't know why; possibly because the installer's validator does more than just validate the key, but also selects which edition to install, so it would require special logic to handle all those extra cases). For example, the installer's key validator will not accept an 8.0 key if the media is 8.1. But the key validation in the actual Windows OS (which is separate from the validator in the
installer) does accept any legitimate key. So it will accept an 8.0 key even if you installed 8.1 straight from 8.1 media. So all you need to do is to toss in an ei.cfg file to tell the installer to shut up and not bother you about the key, and then you can deal with the full key validator in the OS proper instead.
And compared to installing Windows 7 with EFI boot from a USB drive... the process really isn't that different or more difficult. The reason people think it's so much more complicated is the lack of good information about what you need to do and how to do it.