Recovering a badly sysprepped Windows Server 2012 R2 image

I have a Windows Server 2012 R2 image in Azure. It has been curated by a number of people in my org. A lot of software installed and configured etc.

The VM it was created from was then sysprepped without the /generalize parameter.

Now if we create machines from this image the new machines all have the same hostname etc. If we further make changes to a machine created from this image, and then sysprep with the /generalize flag, all the new machines from the second image still miss out on a new machine name from the unattend.xml injected by Windows Azure on machine creation.

How do we recover our image so that the unattend.xml from Azure will run on bootup so our machines get a new name?

thanks!