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Before you do anything, I suggest taking backups of at least one DC from each domain in your forest. Here is a great tutorial for Server 2003.
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How to find out which server holds which role:
netdom query /domain:corp.root fsmo
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Verify Active Directory functionality before you apply the schema extension
repadmin /replsum /bysrc /bydest /sort:delta
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Here is a copy of the support folder from the en_windows_server_2008_r2_with_sp1_x64_dvd_617601.iso which contains the adprep and adprep32 binaries support_from_en_windows_server_2008_r2_with_sp1_x64_dvd_617601.iso.zip
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Required roles of user running /forestprep: Schema Admins group, the Enterprise Admins group, and the Domain Admins group of the domain that hosts the schema master. Windows Reference
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You may need to move the Schema owner role to a new domain controller in order to make it possible for a single user to be in all 3 required groups. This non microsoft site and this Microsoft doc describes how to transfer the Schema owner role to a new domain controller.
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Execute the forestprep on the Schema master:
adprep /forestprep
oradprep32 /forestprep
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Before you can run adprep /domainprep, you must be sure that the updates from /forestprep have replicated to ALL domain controllers in the forest. Below are some helpful commands for checking on the replication.
repadmin /showrepl
repadmin /replsummary
dcdiag
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Execute the following command once for each domain in the forest on the domain's Infrastructure master:
adprep /domainprep
oradprep32 /domainprep