XP Pro OEM to XP Pro Volume License without reinstall

We have a XP Pro Volume license and a previous image of a XP Pro OEM that we did sysprep on. We now have new machines with Vista Business that we are going to continue the XP Pro rollout because of other proprietary software not being Vista compatible. I would prefer not to start a new image from scratch with the Volume License CD, however I will if needed.
Is there a way to go around a reinstall?