Hi,
- Moving drives around won't invalidate your license. It would require you to do things like replace your motherboard, and CPU before your license is invalidated, so you should be fine swapping them, adding more, or even completely replacing them.
- Personally, I always recommend a fresh reinstall, when moving Windows to a new drive, or partition. Reason? Cloning a windows install partition/drive, then booting from the cloned drive can (more often than not) be problematic, and too much of a pain. Reinstalling is always easier in my opinion.
- Risks? You will lose any data that hasn't been backed up, and you will have to reinstall any apps that you previously installed. Also, if you're not using the original install media that came with your computer (restore partition/CDs, depending on your computer), you will likely lose any apps that were preinstalled by your computer manufacturer.
Tip: If you're not using the original install media that came with your computer, you may need to manually go to your computer manufacturer's website to get the latest drivers, and software for Windows on your PC.
Tip2: Install Windows, before you install Linux. This saves you from having to manually specify Linux as your boot loader in the UEFI/BIOS. Linux boot loaders will automatically add an option to boot into Windows, where Windows will not add Linux to it's boot options, forcing you to boot directly into Windows, and doesn't make it easy to add new boot options. I run a dual boot with Windows, and Linux, and this is how I always do it.
Sources: I've been providing technical, and hardware support for Windows PCs (mainly for friends), since the early 2000's. I've also been using Linux regularly for the last 3-4 years as well.
Hope this helps!