I have upgraded 4 computers by now to Windows 10 (clean and full) and noticed an annoying bug (or feature if you like) when additional language packs are installed on your Windows 8.1 Pro OS.
In my case I have 3 languages installed, English (base OS language), Dutch and Italian. When I tried to do a full upgrade on my wife’s laptop I ran into the situation that the Windows 10 upgrade tool wanted to install a Dutch version of Windows 10 and doing so actually disabled the full upgrade possibility. I had a similar problem on my Surface Pro 3. Although I wanted to do a clean upgrade, the tool switched to a Dutch Windows 10 version instead of the installed English base language.
It turns out that the Windows 10 language installed by the Windows 10 upgrade tool is equal to the primary set language and not the base OS language, which is in my case always the English version. The solution is to switch to the base OS language, logoff and logon again, before running the upgrade tool. Now you get all the options including the full upgrade preserving all the installed applications.