I am confused, or your post seems confusing. Are you saying that you initially went from W8 to W10 but when you did that it installed W10 Home not Pro?
Was your initial W8 Pro edition?
You then removed W10 Home, went back to W8 (which edition?? I assume Pro??) and then went from W8 >> W8.1, and then tried to reinstall or re-upgrade to W10 Pro however its only letting you upgrade to W10 Home not Pro?
I am not sure but I don't think you can switch between Home and Pro, someone else may need to chime in.
Your upgrade path seems correct and logical. If I were you, I'd save all files off the Paralells Windows machine, nuke it and start over. Install a fresh W8 Pro Paralells install, then upgrade W8 Pro >> W8.1 Pro, and then go W8.1 Pro to W10 Pro. Are you sure that all of your editions are Pro? Did you ever downgrade from Pro to Home?
Just for grins, open an elevated command prompt (right click on start menu and select Command prompt (admin) or on your start menu right click on Command Prompt and select "run as administrator") and type:
Code:
DISM /online /Get-CurrentEdition
(Gives what edition you are running)
Code:
DISM /online /Get-TargetEditions
(Gives what it can be upgraded to)
I just did this on a W10x64 Pro machine and got:
Code:
(c) 2015 Microsoft Corporation. All rights reserved.
C:\WINDOWS\system32>DISM /online /Get-CurrentEdition
Deployment Image Servicing and Management tool
Version: 10.0.10240.16384
Image Version: 10.0.10240.16384
Current edition is:
Current Edition : Professional
The operation completed successfully.
C:\WINDOWS\system32>DISM /online /Get-TargetEditions
Deployment Image Servicing and Management tool
Version: 10.0.10240.16384
Image Version: 10.0.10240.16384
Editions that can be upgraded to:
Target Edition : Enterprise
Target Edition : Education
The operation completed successfully.
C:\WINDOWS\system32>
Should work on W8 and I know this works on W8.1. You should see "Professional". If not, that's probably your issue.