Many customers are running into the need for a Windows 2008 or newer PKI infrastructure in order to enroll and auto enroll newer client operating systems like Windows 7, Vista, and Windows 2008 Server.
Actually, many business customers found the lack of certificate support in Vista (without upgrading their CA's later) as one of the reasons it wasn't business ready. With Windows 7 being almost 10 years newer than Windows XP, many business customers are ready for a software refresh and Windows 7 has enough other appealing features to help that decision along.
There are basically two routes to go; in place upgrade or migration. The only time I would attempt an in place is on a VM so that a snapshot could easily be taken and rolled back in the case of a failure. A migration gives a fresh start, but requires some additional time to complete because between steps you need to wait for certs to issue to clients.
Because certificates are fairly sensitive information, I won't post screen caps, but rather overview the process.
Research and Design
Research what your existing CA is in use for. Anything it has issued needs to be either determined to be invalid (expired, not in use, not needed) or documented as something to replicate on the new CA. The other decision on design is around what CA architecture and hierarchy you want or need. Depends on the size and complexity of your organization this can differ greatly. For most organizations under 2000 users, I would say a single CA is sufficient, and if an additional are needed, use the PKI planning guides that Microsoft provides, or better yet, read Komar's 2k8 PKI book.
Implement and Re-Issue certs
Depending on your usage, this could take a long time. Audit existing certificates, revoke the ones that are not in use or expired, and start re-issuing them on the new CA architecture. For larger organizations, this may take months to complete. Luckily, you can choose to have both CA's active. I recommend changing the certificate templates on the old CA to read only, and no longer allow enroll and auto enroll as you migrate each template type successfully, this way, the old CA still validates certificates issued that you haven't updated while you can work on updating them, without any noticeable downtime.
Decommission Legacy CA
The "easy" part for sure. Removing a CA (Unlike uninstalling Exchange) there are no checks or audits to make sure you did everything correctly. If you didn't notice that your Cisco ASA or VPN Concentrator had a certificate issued and miss it, it may cause some issues for you. I recommend stopping and disabling your legacy CA for a few days or even weeks (this depends on your comfort level, and organization) before you make the decision to decommission. Even then, before you decommission, I would also really recommend taking a complete backup of the server.
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