No offense intended, this is not Microsoft Store Support and maybe you should use the Microsoft Store Answer Desk to resolve you Microsoft product purchase questions! 
I didn't say, or infer, you where asking VMware directly how to download Windows and how others handle it is somewhat irrelevant since you want to have a separate Virtual Machine for Windows 8 and keep your Windows 7 Virtual Machine then technically you need to purchase a version of Windows that is in essence for a PC without an OS already installed. This is typically referred to as a Full Version not and Upgrade Version. Therefore you need to go to Microsoft, ascertain what's available, in what format and purchase accordingly to what suites your needs/wants/desires and not anyone else's!
but my question has to do with that verbiage on the MS Win site, about 8.1 being for Win 7 users only.
Which is exactly why you should be contacting Microsoft to get clarification on this and is why there is a Microsoft Store Answer Desk to help resolve your Microsoft product purchase questions!
I'd think others would be doing the very same thing (keep an existing VM while creating a new one in the new version), so I don't think it's that irrelevant!
It is irrelevant because whether or not you choose to upgrade or clean build you're not entitled to keep the previous version as a Virtual Machine unless you purchase a Full package and are only entitled to one instance, physical or virtual unless you purchase additional licenses! Therefore when purchasing the Full package you can install it on one machine, physical or virtual but not both and that goes for bit-ness too. Meaning if you purchase a package that contains both 32-bit and 64-bit media you're entitled to install one or the other, not both!
So the bottom line is, acquire the media that is appropriate for your needs/wants/desires and not anyone else's!
Part of it, too, is that Win 7 is such a solid OS, and many people have said what a crazy change Win 8 is... I don't want to just dump 7 for 8. I would think lots of people would want to try out 8 without that commitment.
Again what others do is irrelevant since keeping a copy of a running previous version when using Upgrade Media violates the EULA without purchasing additional licenses of the proper type. If you want to test drive Windows 8 before purchasing then download the Windows 8.1 Enterprise Evaluation and use it first.
I am not trying to make any assumptions as was only trying to help! I stand firm by what I originally said and that was "No offense intended, this is not Microsoft Store Support and maybe you should use the Microsoft Store Answer Desk to resolve you Microsoft product purchase questions! ".
Did you use the link originally provided and open a chat window and discuss it with someone from Microsoft?