How to renew a Root Certificate

That article gives the basics.  A few extra things:
1) In the CA MMC - right click CAName - All Tasks - Backup CA - backup the cert database and the private key.  Then do a full system backup, including system state - hold onto this copy for at least a month, preferably until the cert expires.
2) Note that if your root is expiring, ALL of your certs that chaing to that root are expiring.  CA servers will not issue a cert past its own validity period - it will truncate them once the defined lifetime exceeds its own expiration date.
3) After renewing, make sure to backup the CA database and private key, issue a fresh CRL and backup the server again.
4) If you haven't experienced a CA renewal yet, note that you may notice a (1) appearing at the end of various filenames - this indicates the 1st renewal and is perfectly normal.
5) Remember to re-deploy the root certificate via GPO, etc. for your clients and servers.
6) When dealing with the root, it is best to use a new keyset instead of reusing the same keyset.  Unless you tell it otherwise, this will be default behavior, but I have seen references that tell you to just keep using the same keyset indefinitely, which is a bad thing.  For subordinates, re-keying is fine as long as you use a fresh keyset every so often - usually 5 years is as long as you want to keep it around.
7) It is best practice to renew CA certs years ahead of time.  It is better to have a 10 year cert that you replace after 5 years than to have a 5 year cert you use for 4.9 years.  See #2 - truncated dates.  Each CA cert should be valid for twice the time period of the longest cert it issues and should be replaced 1/2 of its lifetime.  This prevents certs all expiring at the same time.
8) Yes, you will have 2 CRLs to publish until the original cert expires.  If this is scripted to copy *.crl (instead of caname.crl) to your CDP locations, you should be fine.
9) Commonly forgotton: Remember to copy the new cert to each of the AIA locations.
10) If there were any changes to the CDP, AIA, validity period, key strength (should be 2048), etc. - now is the best time to do it.  This can all be set up ahead of time for the registry settings (or use a CApolicy.inf for cmd line renewal) and will be enforced at the time of cert issuance.