Windows Server 2008 R2 Standard to Enterprise Problems

A few months ago I setup a Citrix XenApp cluster running on Windows Server 2008 R2 Standard Edition using the temporary 180 day license key.

Recently the company bought a Windows Server 2008 R2 Enterprise DataCenter license. This means I need to upgrade the Windows edition from Standard to Enterprise.

I attach the disk to the VM and start the upgrade process through XenCenter, it runs through all checks and unpacks all Windows files and seems to create a Windows Setup partition, it then reboots and trys to boot into this partition and I get a blue screen telling me to CHKDSK the hard drive with the following error message:

STOP: 0x0000007B

As XenApp is already setup and working I really do not want to go down the route of rebuilding this server (as I already had to do this once down to issues with XenApp).

The server did have 8GB of RAM assigned to it, I have tried reducing this down to 2GB's as I read this can cause an issue.

Also I can boot back into the Windows Server 2008 R2 Standard partition without any problems.

UPDATE

I have managed to get round the urgency by re-arming the license, giving me another 180 day trial..but would be nice to work out why this is happening!