Hmm... I could have sworn the ISO I have I used on a customers machine and it just activated. They didn't have pro either. But I also know i used it to install Pro. Are you sure all versions are in the same iso?
I don't know if the iso contains more than 1 version.
I will create a partition on my tablet C: drive and try to install from the 8.1 pro iso, which I have from my desktop upgrade.
This is the iso gotten from Microsoft using the "download with product key" procedure.
I guess this is the only way to know for sure.