I've found that sometimes a DC requires a reboot to become a GC.
So, just like Laura said, make the second a GC (see article below), reboot,
allow for replication and then reverse the process on the first...
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http://www.msresource.net/kb/ConfigureGC.html
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Paul Williams
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message Configuring the new GC should be a simple matter of going into AD Sites &
Services, going to the property page of %servername%-->NTDS Settings and
placing a check-mark next to "Global Catalog." But since Murphy was an
optimist, have both machines configured as GCs at the same time for a few
days if at all possible, to see if anything untoward pops up in your Event
Logs. This will also ensure that the old GC has replicated all of its data
to the new one before you decomission it.