If you want to do a clean install with Windows 10 onto the SSD installed in the desktop - you could use the Windows 7 HDD to get the genuineticket.xml file with, install the SSD, clean install Windows 10 to it, activate it by copying the genuineticket.xml to the right folder:
Clean Install Windows 10 Directly without having to Upgrade First - Windows 10 Forums
Don't worry about "using up" your Windows 8 retail license by upgrading the desktop with it. Once the upgrade to Windows 10 is done on the desktop it won't really matter if it came from the Windows 7 license for the desktop or from your Windows 8 license. In other words - the Windows 7 license can still be for the desktop (which would actually be an upgrade to Windows 10) and you can move the Windows 8 retail license to another computer after the upgrade to Windows 10.
backup 1st of the SSD, a Complete Image backup. Use the free program called Macrium Reflect. Store this image on a 2nd drive if you have one. Because as was mentioned earlier, you can't upgrade that SSD using the OS that's installed on it, if you switch computers. But once you make your image backup of that SSD then you can delete everything on the SSD. Make sure you know which one is windows 7 and which is windows 8 when do your image backup, label them so you will know. Hard drives are real cheap these days if you don't have a spare to keep the image backups on. (does not have to be a ssd for storing)
You will also need your serial numbers for both OS's just to be safe. Use this program to find your Windows product keys. If you don't know them. Showkey
Once you have done your image backups, and saved them to a 2nd drive, then you can proceed to upgrade 1 of your computers. Just take it one step at a time. Making your backups, you will not lose any license abilities, on either machine.
OEM license has to stay with the computer its on. Retail license can be used on any computer.
Update: Didn't see NavyLCDR post, my bad Follow his instructions, he is very knowledgeable on this topic!!!
Last edited by OldMike65; 18 Jan 2016 at 09:38.
It is very good advice to make an image first before upgrading - if there is something on the drive that you don't want to take the chance of losing.
Also good advice to make sure you have written down the product keys for the previous operating systems.
A nicer response is 'I had issues, and whilst personally I would stay away from it, I advise you make an image backup, try it for yourself, and then if unhappy, you can revert back'
And what in particular does that explain.