• 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.

• How to find out which server holds which role: netdom query /domain:corp.root fsmo

• Verify Active Directory functionality before you apply the schema extension repadmin /replsum /bysrc /bydest /sort:delta

• 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

• 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

• 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.

• Execute the forestprep on the Schema master: adprep /forestprep or adprep32 /forestprep

• 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

• Execute the following command once for each domain in the forest on the domain's Infrastructure master: adprep /domainprep or adprep32 /domainprep