Has Microsoft never heard of error messages? Why do they write tools that either do nothing, or hang, or quit if the slightest thing isn't right. I suppose if there were an error message they would probably have gotten it wrong--better not to have any--the user will know something is wrong anyway! But seriously ...
All went ok up to the point where I needed to enter my Product key. Windows 10 Setup does not my product key which is from an OEM System Builder Pack which based on this article
sounds like slightly bad news, the comment in the FAQ being
Not all "system builder" products for Windows 7 are currently available on the site. We are working to add more, but for now, we suggest you
contact Support.
which I suppose means waiting until monday. (In any case I was not terribly conscious re my choice of ISO file except to get 64-bit. Since my Home Premium was english only I chose the single-language option. But I didn't even know what the other choices meant.)
I got rid of my physical PC and moved this Windows 7 to Parallels and was able to use the OEM license. Perhaps the Parallels migration tool took care of it. I can't remember what process I used. Why doesn't the setup simply use the existing license for an upgrade?
The license issue brings up another concern which is that I cloned my Windows 7 VM in order to start this whole process. I basically want a Windows 10 trial, having missed the boat on the beta, thanks to Microsoft closing off the trial in advance without prior notice. So what I don't want to happen is to have my originally Windows 7 VM lose its license because I activate a Windows 10 that I don't really (ever) want to use for anything serious. I never intend to use both VMs at the same time, nor do I need files shared between the two. I suspect the VM clone (normal, not linked) produced two VMs with the same machine identification but I don't know the details of that nor whether Microsoft's identification process changed between 7 and 10. Certainly I seem to be able to run either copy of the Windows 7 VM and to go back and forth, at least in cursory testing. I'd appreciate any info you have about all that.