Here is my situation. We have a new virtualized server that is going to be running either Win2k8 R2 server or Win2k11 server (we haven't decided if we will be purchasing a license for SBS or the standard Windows Server). It is going to be replacing our our current Win2k server that is our domain controller and DHCP server, among other things. Is it worth it to try to migrate settings from the old server to the new one on the same domain? Or would I be better off setting everything up from scratch, including creating a new forest?
its probably better in the very least to setup a new forest purely because of the newer features and security functionality given by the newer versions of active directory.
I would generally always recommend a fresh install from scratch as it helps clean out bumf and make you check if you really need that setting or if it is the most efficient way of doing something. often there are times where a new feature will eliminate some of the things you may have needed to do on the old server.
SBS 2011 must be a new forest I believe. If you go Windows Server 2008 R2 then I would just dcpromo it, move FSMO roles, then dcpromo the 2000 box out of the domain. Lastly upgrade your forest/domain to 2008 R2 mode to get the new AD Features. You may have to leap frog through a temp Server 2003 virtual box if you can't get a 2008 R2 box to be a DC in a 2000 forest/domain.
Otherwise if you go from scratch the computers have to be removed and re-added to new domain, users get new passwords, groups are gone, and any apps you have like Exchange won't work. File/folder permissions have to be redone, etc. etc.
New and clean is nice, but not necessarily the fastest way. I've never seen a AD forest get "old" in a bad way. Nearly all forests built in the old days are still around so Microsoft does a great job of keeping it compatible and clean.