MS really seem to have pulled out all the stops in making this a solid and reliable process, and it shows.The only real scenarios in which I could see anything bad happening would be if you lost power partway through (even then I'm not certain), or if your AD was already screwed to begin with (in which case you have bigger problems).If you have more than one DC then make sure there are absolutely no errors reported by . AD will protect itself in most cases from failed schema updates.Make sure you have backups of every AD Database (from each DC). If the LDIF file doesn't pass syntax (say you BSOD in the middle of an update), then it will not be loaded. I've never seen a schema update (so long as it's done properly) go wrong.I hope you didn’t have a DC running a beta product in your production Forest!), the required schema extensions here have already been performed.Standard approach for me would be to ensure that everything is functioning properly beforehand (via dcdiag, replmon, etc), and ensure that I have a known-good backup of AD in case the worst happens.I'd keep this backup for as long as possible, as AD can be so damn robust that problems may not manifest for a long time afterwards.
This should give you an idea of what you’ll see: Screenshot 1 is a Windows Server 2003R2 SP2 Domain Controller; screenshot 2 is a Windows Server 2008R2 SP2 Domain Controller.
Exchange 2007, OCS, SCOM all require schema changes for example, it's not just something that happens when you are considering a major shift from (say) a Windows 2003 to a Windows 2008 infrastructure.