I found this topic because I was facing this problem too.
I was trying to install a Windows 8 Pro on a Asus laptop which was preinstalled with Windows 8 standard.
I resolved this way: add a file called ei.cfg in the directory "sources" of your Windows 8 ISO.
The file must contains those strings:
Under edition you can insert Core, Professional or Enterprise (depends on what you what to install). Under VL (which stands for Volume Licence) you can set 1 (true) or 0 (zero). This probably stand for "Should I use the Volume Licence?". 0 means no, so the installer prompt you with the cd key request!
Save the file and boot from your modded ISO. Install Windows 8 and insert the correct cd key (the pro edition one in your case) when asked.
Maybe it's late for Norm, but can be usefull for others who land there from Google.