My current thinking is:
1) Wait a while. MS is going to be releasing a new upgrade to Win10 that will, for the first time, allow you to do a clean-install of Win10 and THEN manually enter the product key of your previous OS. That is supposed to come out some time next week.
2) Make an image backup of your current setup using a third-party imaging/restoration tool BEFORE doing the upgrade. My recommendation is to use Macrium Reflect. Macrium Reflect (MR) provides a FREE version that can be used to image and restore partitions or entire drives.
What I recommend is the following:
1) Download and install Macrium Reflect (MR)
2) Run MR and choose the option: "Create an image of the partition(s) required to backup and restore Windows" to write a full backup to an external drive or USB stick
3) Use the option to create a boot USB stick or CD
NOW, you have the means to restore a full working system from the external drive or USB stick in only a few minutes.
3) Use the Media Creation Tool to do the Upgrade, not Windows Update:
Installing Windows 10 using the media creation tool - Windows Help
You can either create install media (DVD or USB) or you could choose the option Upgrade this PC now. Either approach will work.
