sysprepping

Hello I have sysprep'ed machine (using out of the box experience + generalize options ), created image using imagex. We have now deployed the image to a couple of machines, and realised we should have removed the computer and domain settings , as it conflicts with an active PC. Is there a way i can load this image and change the computer name, and join to a WORKGROUP instead of Domain, without the need of creating the image from the beginning ? I have loaded the image into WSIM , however when I browse to z86_Microsoft-Windows-UnattendedJoin_6.1.7601.17514_neutral the identification fields cannot be modified (e.g. joinWorkGroup etc etc. please help. thanks