Eric,
Here's the good news..... once a properly activated W7Pro64 install has been updated to W10Pro64 then the UUID (in BIOS) is registered at the central MS servers, and that workstation is considered W10 activated for life. That is, you can do a clean install from scratch on a freshly formatted SSD or HDD and the process will recognize your workstation and proceed with the clean W10Pro64 install.
Here's the bad news.... that is what you'll need to do now. The new BIOS for the Z600, Z800, Z400 works fine with W10, and those all came out 4/16.
Your problem is that something did not translate over well with the W10 update process, and you won't be able to bandaid over the mess. I never update; clean installs in my mind are the only way to ensure high liklihood of a new OS install success.
Of interest, this W10 activation process results in easy capture of an image of an install from one workstation and then clone of that image over to another same-type workstation that is W10 activated. And, even other workstations that are related but not same-type. For example, a xw6400 image cloned onto a xw6600 or xw4600, etc. Have done that.