You plan sounds ok.
Before starting i would run replmon from the run line or repadmin /showrepl,
dcdiag /v and netdiag /v from the command prompt on the existing DC's to
check for errors, if you have some post the complete output from the command
here or solve them first. For this tools you have to install the support\tools\suptools.msi
from the 2003 installation disk.
The FSMO roles should be held by the newest OS DC, so new AD group for example
will be created. ( http://www.yqcomputer.com/
Switch to AD integrated DNS zones if not done.
Also run adprep /rodcprep to prepare for read-only domain controllers.
Make all DC's GC in a single forest domain.
During promotion configure a fixed ip and set the preferred DNS server to
one older DC/DNS server only, after everything is replicated set itself as
preferred and another domain DNS as secondary.
Do not forget to reconfigure client DNS settings for the new DC/DNS.
After promotion run again the diagnostic tools, on the 2008 copy the netdiag
from a 2003 DC, rest is included.
To check for problems just disconnect the old DC for some days from the network,
if none exist reconnect and then demote it.
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