Updated: Misread the earlier version; updating to reflect further research.
According to the Windows 10 Enterprise FAQ for IT Professionals, you should be able to perform an in-place upgrade:
Can I upgrade computers from Windows 7 or Windows 8.1 without deploying a new image?
Computers running Windows 7 or Windows 8.1 can be upgraded directly to Windows 10 through the in-place upgrade process without a need to reimage the device. You can use System Center Configuration Manager to manage this upgrade automatically through an Operating System Deployment task sequence. See How to upgrade to Windows 10 using the task sequence in System Center 2012 R2 Configuration Manager on the System Center Configuration Manager Team Blog.
(See the second question in the Administration section)
I've looked at a few different public support forums for related reports and there do not seem to be a widespread problems with inline updates for Win10 Enterprise evaluation. I did find one report mentioning that it seems to occur when choosing different country/locale settings than were originally selected.
Traditionally, in-place updates become possible later in the development cycle. Depending on when you downloaded your .ISO (or when it was last updated on your source), it's possible that in-place upgrade is not fully complete.
Since a new evaluation version is either imminent or shortly to be released, perhaps trying again later will provide a cleaner experience.
For those attempting to install the "free upgrade" binaries, per the Windows 10 FAQ:
"Some editions are excluded: Windows 7 Enterprise, Windows 8/8.1 Enterprise, and Windows RT/RT 8.1. Active Software Assurance customers in volume licensing have the benefit to upgrade to Windows 10 enterprise offerings outside of this offer.
(See the answer to "What edition of Windows will I get as part of this free upgrade?")
In other words, I don't think you'll be able to upgrade Enterprise using the free upgrade installation binaries.
Hope this helps...