I've always been prompted for a product key when performing an upgrade from a previous qualifying version of Windows.
I've always just chosen skip or do this later or I don't have a product key and continued with the upgrade. Assuming that the previous version was activated and genuine, I've never had a subsequent problem with Windows 10 activating
I noticed that with the latest ISO, the two previous prompts have been eliminated where you could choose skip and or do this later and only the I don't have a product key seems to remain.