I upgraded a 5.1.2 PDC server to 5.6. The upgrade went well except that all the clients (8 XP Pro boxes) could no longer authenticate against the PDC. Only users with a cached copy could log on. In computer management, SME accounts in XP groups appeared as numbers...just like when a computer can't see a domain controller. I think the upgrade to 5.6 changed the "sid" or something significant. If you have a PDC running with XP clients, do some testing first.
Anyone else experience problems like this? I really need to understand this.
ryan