In addition to vip32’s suggestions for offline editing of an image:
The Resetting Windows Activation section of the TechNet article ‘How Sysprep Works’ explains that while sysprep can be run any number of times, the clock for Windows Product Activation can only be reset three times. If you have reached the limit, then you will have to rebuild your master from scratch. If you have not reached the limit, you can re-run sysprep and re-capture the image for deployment.
When you run sysprep with the /Generalize switch, which is required before capturing the final image, the activation clock is reset.
To clarify the unsupported sysprep scenarios, you cannot clone a non-sysprepped image to another machine, and then run sysprep as the final step in the deployment. The image you deploy must be sysprepped prior to deployment. You can, however, deploy your sysprepped image, make changes, run sysprep again, and then re-capture the image for deployment. However, as mentioned above, you will eventually run into the activation limit if you do this too many times. You can work around this limit by setting up your master on a virtual machine and capturing a snapshot of the image just before you run sysprep. If you need to make any changes after this, simply re-load the snapshot, make your changes, re-capture a new snapshot and then run sysprep, and capture the new image for deployment.
Hope this helps,