Start by making sure that you make a full backup of the DC holding the Schema Master role. Apply R2 Schema update early to allow replication to propagate to DCs in the whole forest. Note that any new attributes can be disabled (not deleted) if they are not needed. In the worst case, you could do an authoritive restore of AD but that is not common.
If possible, run the test upgrade on a non-production / pre-stage environment that is representative of the actual AD forest. One way is to do a P2V or add a VM and setup as DC then remove from production environment to test. Also, use the OCS best practice analyzer to help you check the health and readiness of your infrastructure.
So far, no reported incidents running AD schema update that I am aware of (for Exchange 2007, OCS 2007 R1 to R2 RTM) but precautionary measures should always be planned and executed as a best practice.
Please let us know if this helps. THanks.
TechNet Forum Moderator - http://www.leedesmond.com/weblog/