When updating to a new build of Windows 10 Technical Preview Windows Enterprise must be reactivated. There are some Microsoft product keys on the net for various versions of Win10 TP.
With the latest build (10240) these keys no longer work, it seems Microsoft has disabled the online activation until the end of July 2015.
Here's a workaround how to activate Windows 10 Enterprise:
winaero.com/blog: How to activate Windows 10 build 10240
The workaround uses the Slmgr-Script and WMI. For details see the page Slmgr.vbs Options for Volume Activation on Microsoft TechNet.
Follow the steps here:
Open a elevated command prompt and execute
slmgr /ipk W269N-WFGWX-YVC9B-4J6C9-T83GX
...even if this command returns an error. Now execute this:
slmgr /skms kms.xspace.in
Finally run this command:
Now Windows 10 Enterprise TP Build 10240 should be activated.
This is no official workaround, just found it on the Internet here. At least for my Windows 10 Technical Preview this worked (and helped because I want to continue working for the next two weeks with my machine without reinstalling or downgrading...).
Hope this helps other Windows 10 Insiders who installed the Enterprise version - till the RTM is out.